memory_limit

memory_limit is a configuration directive of the PHP language that sets the maximum amount of RAM a single script may consume during one request. When a script exceeds this threshold, PHP halts its execution and throws a fatal "Allowed memory size exhausted" error. This mechanism protects the server from a situation where one inefficient or malicious script would drain the entire machine's memory.

How memory_limit works

The value is expressed in bytes or with a unit suffix, for example 128M or 256M. The limit applies separately to each process handling a request — if the server processes dozens of requests in parallel, each has its own allowance, so the value must be chosen with the available RAM and the number of PHP-FPM processes in mind. The currently effective value can be checked in the output of the phpinfo() function. A setting of -1 means no limit, which is discouraged on production servers.

Practical application

The default 128 MB is enough for simple sites, but larger applications — Magento, stores with many plugins, importing large files or generating PDFs — may need 256 MB or more. The limit is changed in php.ini, in the PHP-FPM pool configuration, via a php_value directive in the .htaccess file (on servers with mod_php), or with ini_set() in code.

On shared hosting the upper bound of memory_limit is set by the provider so that one customer cannot dominate the resources of a shared machine. On a VPS or dedicated server the administrator has full control over this value. It is worth remembering that raising the limit treats the symptom rather than the cause — if a script uses excessive memory, a better solution is to optimize it, for example by processing data in smaller chunks.