PHP loads all available functions into a giant hashset (see zend_disable_function in PHP's source). 'disable_functions' removes the functions from the hashset,

PHP loads all available functions into a giant hashset (see zend_disable_function in PHP's source). 'disable_functions' removes the functions from the hashset, making them unavailable to be called. Due to its interpreted nature, this indirection works much like a sandbox would.