If I understand correctly, the php.ini parameter zend.enable_gc (see http://www.php.net/manual/en/info.configuration.php) determines whether garbage collection is enabled:
<?php
var_dump(gc_enabled());
ini_set('zend.enable_gc', 0);
var_dump(gc_enabled());
?>
In PHP 5.3.0 this prints:
bool(true)
bool(false)
Oh, and of course gc_enable() and gc_disable() turn on or off the collection of garbage.