... Accepting Arguments . ... If your function accepts arguments passed by reference that you intend to modify, you need to take some precautions. What we didn't say yet is that under the circumstances presented so far, you don't have write access to any zval containers designating function parameters that have been passed to you. ... Zend deals with this situation in a special way: Whenever a parameter to a function is passed by reference, it performs automatic zval separation. ...