Question

I am programming a web API client in PHP that parses CSV data into associative arrays and I want to protect my users from data-duplication when using these arrays.

My users will never be writing to these arrays (theoretically they could but it makes no sense in practice).

Now my question is… if my users pass these arrays around as arguments to methods, will PHP’s copy-on-write mechanism prevent data-duplication or will any method that doesn’t explicitly accept a reference to an array receive a complete copy of the array?

Answer 1

Copy on write as the name suggests means no variable is being copied until something is written; as long as not a single byte is changed in the variable passed around, PHP takes care of avoiding unnecesary duplicates automatically and without the need of using explicit references thanks to this mechanism.

This article explains in detail how is this implemented in the source code of PHP, and as the article suggests, using xdebug one can easily check the variables are not being duplicated with the function xdebug_debug_zval.

Additionally this answer here on SO has more on Copy-on-Write.