Creating Variables

Overview

To create new variables that can be seen "from the outside" by the executing script, you need to allocate a new zval container, fill this container with meaningful values, and then introduce it to Zend's internal symbol table. This basic process is common to all variable creations:

zval *new_variable; 

/* allocate and initialize new container */
MAKE_STD_ZVAL(new_variable); 

/* set type and variable contents here, see the following sections */ 

/* introduce this variable by the name "new_variable_name" into the symbol table */
ZEND_SET_SYMBOL(EG(active_symbol_table), "new_variable_name", new_variable); 

/* the variable is now accessible to the script by using $new_variable_name */

The macro MAKE_STD_ZVAL allocates a new zval container using ALLOC_ZVAL and initializes it using INIT_ZVAL. As implemented in Zend at the time of this writing, initializing means setting the reference count to 1 and clearing the is_ref flag, but this process could be extended later - this is why it's a good idea to keep using MAKE_STD_ZVAL instead of only using ALLOC_ZVAL. If you want to optimize for speed (and you don't have to explicitly initialize the zval container here), you can use ALLOC_ZVAL, but this isn't recommended because it doesn't ensure data integrity.

ZEND_SET_SYMBOL takes care of introducing the new variable to Zend's symbol table. This macro checks whether the value already exists in the symbol table and converts the new symbol to a reference if so (with automatic deallocation of the old zval container). This is the preferred method if speed is not a crucial issue and you'd like to keep memory usage low.

Note that ZEND_SET_SYMBOL makes use of the Zend executor globals via the macro EG. By specifying EG(active_symbol_table), you get access to the currently active symbol table, dealing with the active, local scope. The local scope may differ depending on whether the function was invoked from within a function.

If you need to optimize for speed and don't care about optimal memory usage, you can omit the check for an existing variable with the same value and instead force insertion into the symbol table by using zend_hash_update():

zval *new_variable;

/* allocate and initialize new container */
MAKE_STD_ZVAL(new_variable);

/* set type and variable contents here, see the following sections */

/* introduce this variable by the name "new_variable_name" into the symbol table */
zend_hash_update(
    EG(active_symbol_table),
    "new_variable_name",
    strlen("new_variable_name") + 1,
    &new_variable,
    sizeof(zval *),
    NULL
);

The variables generated with the snippet above will always be of local scope, so they reside in the context in which the function has been called. To create new variables in the global scope, use the same method but refer to another symbol table:

zval *new_variable;
     
// allocate and initialize new container
MAKE_STD_ZVAL(new_variable);

//
// set type and variable contents here
//

// introduce this variable by the name "new_variable_name" into the global symbol table
ZEND_SET_SYMBOL(&EG(symbol_table), "new_variable_name", new_variable);

Note: The active_symbol_table variable is a pointer, but symbol_table is not. This is why you have to use EG(active_symbol_table) and &EG(symbol_table) as parameters to ZEND_SET_SYMBOL - it requires a pointer.

Similarly, to get a more efficient version, you can hardcode the symbol table update:

zval *new_variable;

// allocate and initialize new container
MAKE_STD_ZVAL(new_variable);

//
// set type and variable contents here
//

// introduce this variable by the name "new_variable_name" into the global symbol table
zend_hash_update(
    &EG(symbol_table),
    "new_variable_name",
    strlen("new_variable_name") + 1,
    &new_variable,
    sizeof(zval *),
    NULL
);

Note: You can see that the global variable is actually not accessible from within the function. This is because it's not imported into the local scope using global $global_variable; in the PHP source.

ZEND_FUNCTION(variable_creation)
{
    zval *new_var1, *new_var2;

    MAKE_STD_ZVAL(new_var1);
    MAKE_STD_ZVAL(new_var2);

    ZVAL_LONG(new_var1, 10);
    ZVAL_LONG(new_var2, 5);

    ZEND_SET_SYMBOL(EG(active_symbol_table), "local_variable", new_var1);
    ZEND_SET_SYMBOL(&EG(symbol_table), "global_variable", new_var2);

    RETURN_NULL();

}

Longs (Integers)

Now let's get to the assignment of data to variables, starting with longs. Longs are PHP's integers and are very simple to store. Looking at the zval.value container structure discussed earlier in this chapter, you can see that the long data type is directly contained in the union, namely in the lval field. The corresponding type value for longs is IS_LONG (see 46-10).

zval *new_long;

MAKE_STD_ZVAL(new_long);

new_long->type = IS_LONG;
new_long->value.lval = 10;

zval *new_long;

MAKE_STD_ZVAL(new_long);
ZVAL_LONG(new_long, 10);

Doubles (Floats)

Doubles are PHP's floats and are as easy to assign as longs, because their value is also contained directly in the union. The member in the zval.value container is dval; the corresponding type is IS_DOUBLE.

zval *new_double;

MAKE_STD_ZVAL(new_double);

new_double->type = IS_DOUBLE;
new_double->value.dval = 3.45;

zval *new_double;

MAKE_STD_ZVAL(new_double);
ZVAL_DOUBLE(new_double, 3.45);

Strings

Strings need slightly more effort. As mentioned earlier, all strings that will be associated with Zend's internal data structures need to be allocated using Zend's own memory-management functions. Referencing of static strings or strings allocated with standard routines is not allowed. To assign strings, you have to access the structure str in the zval.value container. The corresponding type is IS_STRING:

zval *new_string;
char *string_contents = "This is a new string variable";

MAKE_STD_ZVAL(new_string);

new_string->type = IS_STRING;
new_string->value.str.len = strlen(string_contents);
new_string->value.str.val = estrdup(string_contents);

zval *new_string;
char *string_contents = "This is a new string variable";

MAKE_STD_ZVAL(new_string);
ZVAL_STRING(new_string, string_contents, 1);

If you want to truncate the string at a certain position or you already know its length, you can use ZVAL_STRINGL(zval, string, length, duplicate), which accepts an explicit string length to be set for the new string. This macro is faster than ZVAL_STRING and also binary-safe.

To create empty strings, set the string length to 0 and use empty_string as contents:

new_string->type = IS_STRING;
new_string->value.str.len = 0;
new_string->value.str.val = empty_string;

MAKE_STD_ZVAL(new_string);
ZVAL_EMPTY_STRING(new_string);

Booleans

Booleans are created just like longs, but have the type IS_BOOL. Allowed values in lval are 0 and 1:

zval *new_bool;

MAKE_STD_ZVAL(new_bool);

new_bool->type = IS_BOOL;
new_bool->value.lval = 1;

Arrays

Arrays are stored using Zend's internal hash tables, which can be accessed using the zend_hash_*() API. For every array that you want to create, you need a new hash table handle, which will be stored in the ht member of the zval.value container.

There's a whole API solely for the creation of arrays, which is extremely handy. To start a new array, you call array_init().

zval *new_array;

MAKE_STD_ZVAL(new_array);

array_init(new_array);

To add new elements to the array, you can use numerous functions, depending on what you want to do. 46-8, 46-9 and 46-10 describe these functions. All functions return FAILURE on failure and SUCCESS on success.

All these functions provide a handy abstraction to Zend's internal hash API. Of course, you can also use the hash functions directly - for example, if you already have a zval container allocated that you want to insert into an array. This is done using zend_hash_update() for associative arrays (see 46-11) and zend_hash_index_update() for indexed arrays (see 46-12):

zval *new_array, *new_element;
char *key = "element_key";
      
MAKE_STD_ZVAL(new_array);
MAKE_STD_ZVAL(new_element);

array_init(new_array);

ZVAL_LONG(new_element, 10);

if(zend_hash_update(new_array->value.ht, key, strlen(key) + 1, (void *)&new_element, sizeof(zval *), NULL) == FAILURE)
{
    // do error handling here
}

zval *new_array, *new_element;
int key = 2;

MAKE_STD_ZVAL(new_array);
MAKE_STD_ZVAL(new_element);

array_init(new_array);

ZVAL_LONG(new_element, 10);

if(zend_hash_index_update(new_array->value.ht, key, (void *)&new_element, sizeof(zval *), NULL) == FAILURE)
{
    // do error handling here
}

To emulate the functionality of add_next_index_*(), you can use this:

zend_hash_next_index_insert(ht, zval **new_element, sizeof(zval *), NULL)

Note: To return arrays from a function, use array_init() and all following actions on the predefined variable return_value (given as argument to your exported function; see the earlier discussion of the call interface). You do not have to use MAKE_STD_ZVAL on this.

Tip: To avoid having to write new_array->value.ht every time, you can use HASH_OF(new_array), which is also recommended for compatibility and style reasons.

Objects

Since objects can be converted to arrays (and vice versa), you might have already guessed that they have a lot of similarities to arrays in PHP. Objects are maintained with the same hash functions, but there's a different API for creating them.

To initialize an object, you use the function object_init():

zval *new_object;

MAKE_STD_ZVAL(new_object);

if(object_init(new_object) != SUCCESS)
{
    // do error handling here
}

Resources

Resources are a special kind of data type in PHP. The term resources doesn't really refer to any special kind of data, but to an abstraction method for maintaining any kind of information. Resources are kept in a special resource list within Zend. Each entry in the list has a correspondending type definition that denotes the kind of resource to which it refers. Zend then internally manages all references to this resource. Access to a resource is never possible directly - only via a provided API. As soon as all references to a specific resource are lost, a corresponding shutdown function is called.

For example, resources are used to store database links and file descriptors. The de facto standard implementation can be found in the MySQL module, but other modules such as the Oracle module also make use of resources.

To create a new resource you need to register a resource destruction handler for it. Since you can store any kind of data as a resource, Zend needs to know how to free this resource if its not longer needed. This works by registering your own resource destruction handler to Zend which in turn gets called by Zend whenever your resource can be freed (whether manually or automatically). Registering your resource handler within Zend returns you the resource type handle for that resource. This handle is needed whenever you want to access a resource of this type later and is most of time stored in a global static variable within your extension. There is no need to worry about thread safety here because you only register your resource handler once during module initialization.

The Zend function to register your resource handler is defined as:

ZEND_API int zend_register_list_destructors_ex(rsrc_dtor_func_t ld, rsrc_dtor_func_t pld, char *type_name, int module_number);

There are two different kinds of resource destruction handlers you can pass to this function: a handler for normal resources and a handler for persistent resources. Persistent resources are for example used for database connection. When registering a resource, either of these handlers must be given. For the other handler just pass NULL.

zend_register_list_destructors_ex() accepts the following parameters:

The resource destruction handler (either normal or persistent resources) has the following prototype:

void resource_destruction_handler(zend_rsrc_list_entry *rsrc TSRMLS_DC);

typedef struct _zend_rsrc_list_entry {
     
    void *ptr;
    int type;
    int refcount;

} zend_rsrc_list_entry;

Now we know how to start things, we define our own resource we want register within Zend. It is only a simple structure with two integer members:

typedef struct {
     
    int resource_link;
    int resource_type;

} my_resource;

void my_destruction_handler(zend_rsrc_list_entry *rsrc TSRMLS_DC) {

    // You most likely cast the void pointer to your structure type

    my_resource *my_rsrc = (my_resource *) rsrc->ptr;

    // Now do whatever needs to be done with you resource. Closing
    // Files, Sockets, freeing additional memory, etc.
    // Also, don't forget to actually free the memory for your resource too!

    do_whatever_needs_to_be_done_with_the_resource(my_rsrc);
}

Now that we have defined

1. what our resource is and

2. our resource destruction handler

1. create a global variable within the extension holding the resource ID so it can be accessed from every function which needs it

2. define the resource name

3. write the resource destruction handler

4. and finally register the handler

// Somewhere in your extension, define the variable for your registered resources.
    // If you wondered what 'le' stands for: it simply means 'list entry'.
    static int le_myresource;

    // It's nice to define your resource name somewhere
    #define le_myresource_name  "My type of resource"

    [...]

    // Now actually define our resource destruction handler
    void my_destruction_handler(zend_rsrc_list_entry *rsrc TSRMLS_DC) {

        my_resource *my_rsrc = (my_resource *) rsrc->ptr;
        do_whatever_needs_to_be_done_with_the_resource(my_rsrc);
    }

    [...]

    PHP_MINIT_FUNCTION(my_extension) {

        // Note that 'module_number' is already provided through the
        // PHP_MINIT_FUNCTION() function definition.

        le_myresource = zend_register_list_destructors_ex(my_destruction_handler, NULL, le_myresource_name, module_number);

        // You can register additional resources, initialize
        // your global vars, constants, whatever.
    }

To actually register a new resource you use can either use the zend_register_resource() function or the ZEND_REGISTER_RESOURE() macro, both defined in zend_list.h . Although the arguments for both map 1:1 it's a good idea to always use macros to be upwards compatible:

int ZEND_REGISTER_RESOURCE(zval *rsrc_result, void *rsrc_pointer, int rsrc_type);

What is really going on when you register a new resource is it gets inserted in an internal list in Zend and the result is just stored in the given zval * container:

rsrc_id = zend_list_insert(rsrc_pointer, rsrc_type);
     
    if (rsrc_result) {
        rsrc_result->value.lval = rsrc_id;
        rsrc_result->type = IS_RESOURCE;
    }

    return rsrc_id;

RETURN_RESOURCE(rsrc_id)

Zend now keeps track of all references to this resource. As soon as all references to the resource are lost, the destructor that you previously registered for this resource is called. The nice thing about this setup is that you don't have to worry about memory leakages introduced by allocations in your module - just register all memory allocations that your calling script will refer to as resources. As soon as the script decides it doesn't need them anymore, Zend will find out and tell you.

Now that the user got his resource, at some point he is passing it back to one of your functions. The value.lval inside the zval * container contains the key to your resource and thus can be used to fetch the resource with the following macro: ZEND_FETCH_RESOURCE:

ZEND_FETCH_RESOURCE(rsrc, rsrc_type, rsrc_id, default_rsrc_id, resource_type_name, resource_type)

To force removal of a resource from the list, use the function zend_list_delete(). You can also force the reference count to increase if you know that you're creating another reference for a previously allocated value (for example, if you're automatically reusing a default database link). For this case, use the function zend_list_addref(). To search for previously allocated resource entries, use zend_list_find(). The complete API can be found in zend_list.h.

Macros for Automatic Global Variable Creation

In addition to the macros discussed earlier, a few macros allow easy creation of simple global variables. These are nice to know in case you want to introduce global flags, for example. This is somewhat bad practice, but Table 46-12 describes macros that do exactly this task. They don't need any zval allocation; you simply have to supply a variable name and value.

Creating Constants

Zend supports the creation of true constants (as opposed to regular variables). Constants are accessed without the typical dollar sign ($) prefix and are available in all scopes. Examples include TRUE and FALSE, to name just two.

To create your own constants, you can use the macros in 46-13. All the macros create a constant with the specified name and value.

You can also specify flags for each constant:

• CONST_CS - This constant's name is to be treated as case sensitive.

• CONST_PERSISTENT - This constant is persistent and won't be "forgotten" when the current process carrying this constant shuts down.

// register a new constant of type "long"
     REGISTER_LONG_CONSTANT("NEW_MEANINGFUL_CONSTANT", 324, CONST_CS |
     CONST_PERSISTENT);