Compare PHP Arrays Using Memory References

Question

Is it possible to see if two array variables point to the same memory location? (they are the same array)

Answer 1

Actually, this can be done. Through a php extension.

File: config.m4

PHP_ARG_ENABLE(test, whether to enable test Extension support, [ --enable-test   Enable test ext support])

if test "$PHP_TEST" = "yes"; then
  AC_DEFINE(HAVE_TEST, 1, [Enable TEST Extension])
  PHP_NEW_EXTENSION(test, test.c, $ext_shared)
fi

File: php_test.h

#ifndef PHP_TEST_H
#define PHP_TEST_H 1

#define PHP_TEST_EXT_VERSION "1.0"
#define PHP_TEST_EXT_EXTNAME "test"

PHP_FUNCTION(getaddress4);
PHP_FUNCTION(getaddress);

extern zend_module_entry test_module_entry;
#define phpext_test_ptr &test_module_entry

#endif

File: test.c

#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

#include "php.h"
#include "php_test.h"

ZEND_BEGIN_ARG_INFO_EX(func_args, 1, 0, 0)
ZEND_END_ARG_INFO()

static function_entry test_functions[] = {
    PHP_FE(getaddress4, func_args)
    PHP_FE(getaddress, func_args)
    {NULL, NULL, NULL}
};

zend_module_entry test_module_entry = {
#if ZEND_MODULE_API_NO >= 20010901
    STANDARD_MODULE_HEADER,
#endif
    PHP_TEST_EXT_EXTNAME,
    test_functions,
    NULL,
    NULL,
    NULL,
    NULL,
    NULL,
#if ZEND_MODULE_API_NO >= 20010901
    PHP_TEST_EXT_VERSION,
#endif
    STANDARD_MODULE_PROPERTIES
};

#ifdef COMPILE_DL_TEST
ZEND_GET_MODULE(test)
#endif

PHP_FUNCTION(getaddress4)
{
    zval *var1;
    zval *var2;
    zval *var3;
    zval *var4;
    char r[500];
    if( zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "aaaa", &var1, &var2, &var3, &var4) == FAILURE ) {
      RETURN_NULL();
    }
    sprintf(r, "\n%p - %p - %p - %p\n%p - %p - %p - %p", var1, var2, var3, var4, Z_ARRVAL_P(var1), Z_ARRVAL_P(var2), Z_ARRVAL_P(var3), Z_ARRVAL_P(var4) );
    RETURN_STRING(r, 1);
}

PHP_FUNCTION(getaddress)
{
    zval *var;
    char r[100];
    if( zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "a", &var) == FAILURE ) {
      RETURN_NULL();
    }
    sprintf(r, "%p", Z_ARRVAL_P(var));
    RETURN_STRING(r, 1);
}

Then all you have to do is phpize it, config it, and make it. Add a "extension=/path/to/so/file/modules/test.so" to your php.ini file. And finally, restart the web server, just in case.

<?php
  $x = array("123"=>"123");
  $w = $x;
  $y = $x;
  $z = &$x;
  var_dump(getaddress4($w,$x,$y,$z));
  var_dump(getaddress($w));
  var_dump(getaddress($x));
  var_dump(getaddress($y));
  var_dump(getaddress($z));
?>

Returns(at least for me, your memory addresses will probably be different)

string '
0x9efeb0 - 0x9effe0 - 0x9ef8c0 - 0x9efeb0
0x9efee0 - 0x9f0010 - 0x9ed790 - 0x9efee0' (length=84)

string '0x9efee0' (length=8)

string '0x9f0010' (length=8)

string '0x9ed790' (length=8)

string '0x9efee0' (length=8)

Thanks to Artefacto for pointing this out, but my original code was passing the arrays by value, so thereby was recreating arrays including the referenced-one, and giving you bad memory values. I have since changed the code to force all params to be passed by reference. This will allow references, arrays, and object, to be passed in unmolested by the php engine. $w/$z are the same thing, but $w/$x/$y are not. The old code, actually showed the reference breakage and the fact that the memory addresses would change or match when all variables were passed in vs multiple calls to the same function. This was because PHP would reuse the same memory when doing multiple calls. Comparing the results of the original function would be useless. The new code should fix this problem.

FYI - I'm using php 5.3.2.