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Asked: 2026-05-10T16:51:50+00:00

In PHP, function parameters can be passed by reference by prepending an ampersand to

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In PHP, function parameters can be passed by reference by prepending an ampersand to the parameter in the function declaration, like so:

function foo(&$bar) {     // ... }

Now, I am aware that this is not designed to improve performance, but to allow functions to change variables that are normally out of their scope.

Instead, PHP seems to use Copy On Write to avoid copying objects (and maybe also arrays) until they are changed. So, for functions that do not change their parameters, the effect should be the same as if you had passed them by reference.

However, I was wondering if the Copy On Write logic maybe is shortcircuited on pass-by-reference and whether that has any performance impact.

ETA: To be sure, I assume that it’s not faster, and I am well aware that this is not what references are for. So I think my own guesses are quite good, I’m just looking for an answer from someone who really knows what’s definitely happening under the hood. In five years of PHP development, I’ve always found it hard to get quality information on PHP internals short from reading the source.

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  1. The Zend Engine uses copy-on-write, and when you use a reference yourself, it incurs a little extra overhead. Can only find this mention at time of writing though, and comments in the manual contain other links.

    (EDIT) The manual page on Objects and references contains a little more info on how object variables differ from references.

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