memory_limit

memory_limit is the directive that caps the maximum amount of memory that PHP can use.

The limit is expressed in bytes, although options are available for kb K, Mb M and Gb G, case insensitive.

The limit may be lifted by setting this directive to -1.

PHP requires a minimum of memory when starting. Usually, it is about 2Mb of RAM. Setting any positive value for memory_limit below that threshold prevents PHP from starting.

The default value of memory_limit is 128 Mb.

memory_limit may be changed during execution with ini_set().

<?php

    echo ini_get('memory_limit');  //128M

?>

Documentation

See also PHP memory_limit – understanding and increasing.

Related : Memory, php://memory, php://tmp, Vertical Scaling