This means that whenever you 're calling a function like this in PHP, Zend will automatically ensure that $parameter is being passed as an isolated value, rendering it to a write-safe state:
To change parameters, however, write access is required. Zend deals with this situation in a special way: Whenever a parameter to a function is passed by reference, it performs automatic zval separation. This means that whenever you're calling a function like this in PHP, Zend will automatically ensure that $parameter is being passed as an isolated value, rendering it to a write-safe state: