Fatal error: Call to undefined function xdebug_debug_zval()?

[Rapbox]

When i tried to execute the following code to understand the memory allocation in Php, i got the following error.\[code\]$a = "new string";xdebug_debug_zval( 'a' );$c = $b = $a;xdebug_debug_zval( 'a' );xdebug_debug_zval( 'b' );xdebug_debug_zval( 'c' );unset( $b );xdebug_debug_zval( 'a' );xdebug_debug_zval( 'b' );xdebug_debug_zval( 'c' );$c = $c . 1;xdebug_debug_zval( 'a' );xdebug_debug_zval( 'c' );\[/code\]\[quote\] Fatal error: Call to undefined function xdebug_debug_zval()\[/quote\]What should i have to install to make it work, i just copied from the link http://us3.php.net/manual/en/features.gc.refcounting-basics.phpI am using ubuntu and php 5