Essential Books

Memory Usage

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PHP’s runtime memory limit is set through the INI directive memory_limit. This setting prevents any single execution of PHP from using up too much memory, exhausting it for other scripts and system software. The memory limit defaults to 128M and can be changed in the php.ini file or at runtime. It can be set to have no limit, but this is generally considered bad practice.

The exact memory usage used during runtime can be determined by calling memory_get_usage(). It returns the number of bytes of memory allocated to the currently running script. As of PHP 5.2, it has one optional boolean parameter to get the total allocated system memory, as opposed to the memory that’s actively being used by PHP. 

<?php
echo memory_get_usage() . "\n";
// Outputs 350688 (or similar, depending on system and PHP version)

// Let's use up some RAM
$array = array_fill(0, 1000, 'abc');

echo memory_get_usage() . "\n";
// Outputs 387704

// Remove the array from memory
unset($array);

echo memory_get_usage() . "\n";
// Outputs 350784

Now memory_get_usage gives you memory usage at the moment it is run. Between calls to this function you may allocate and deallocate other things in memory. To get the maximum amount of memory used up to a certain point, call memory_get_peak_usage(). 

<?php
echo memory_get_peak_usage() . "\n";
// 385688
$array = array_fill(0, 1000, 'abc');
echo memory_get_peak_usage() . "\n";
// 422736
unset($array);
echo memory_get_peak_usage() . "\n";
// 422776

Notice the value will only go up or stay constant.

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Performance:
* Performance
* Profiling with Xdebug
* Memory Usage
* Profiling with XHProf
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