PHP/Performance
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PHP, like every programming language implementation, has certain performance characteristics. Being aware of them and coding consistently can help improve web application performance. Some practices provide relatively small gains, but can make a difference when used regularly. Many teams like to include these practices in their coding standards.
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[edit] Code Organization
require_once and include_once can be relatively expensive operations. Only include the files which are required for a given set of code, as opposed to always including all source code files within every other file of an entire project.
Relative paths also make the include and require statements more expensive by forcing the PHP runtime to resolve the path. Use absolute paths whenever possible.
The expense of include and require statements is mostly mitigated by using APC, which will keep the compiled versions of unmodified PHP files in memory for quick reuse.
[edit] Strings
PHP has four types of strings declared within code. Strings within double quotes and heredoc have their variables evaluated (or "expanded") and many escape sequences interpreted. Strings within single quotes and nowdoc do not cause any variable evaluation and only support escaping of single quotes and backslashes. Therefore double quotes and heredoc require more evaluation and processing. When no variable evaluation and no special characters are required, using single quotes will provide slightly better performance. Starting with late versions of PHP 5.2 string performance has been enhanced to the point where using single quotes instead of double is basically irrelevant.
Inline string concatenation is supported in PHP with the period character. Using this simple inline concatenation can require multiple memory allocations. Using the implode function requires only one memory allocation and can therefore be more efficient.
Regular expressions are relatively expensive to process. They require complex parsing and tokenizing, and sometimes require recursion. When possible, use simple string functions instead, such as str_replace and strstr.
[edit] Memory Consumption
Use simple tools, like PHP's built-in functions memory_get_usage and memory_get_peak_usage, to gather memory consumption characteristics. Using these within an application can trap segments of code which are problematic. Comparing memory usage before and after a section of code, then logging these values, will provide accurate metrics.
Implicit garbage collection occurs (basically) when a variable falls out of scope, such as at the end of a function or script. Explicitly unsetting a variable can free its memory before PHP does automatically. This is especially useful in long running scripts, such as some cron jobs, which can explicitly free some memory used early in a script before waiting for the task to complete.
Querying a database can also incur extra memory usage as result sets may be stored in memory until a script is complete. Depending on the database library and how it's used, complete result sets may be brought into memory before they are used by the PHP script. Explicit functions, such as mysqli_free_result may be used to relinquish that memory once it's no longer needed.
When using the Smarty templating engine, expect a memory usage spike when a template is first compiled. Following it's compilation upon first use or after an edit, this spike won't occur.
