PHP Native Function
PHP defines its own native functions. They are part of the core extensions, which are always compiled, or extensions.
PHP native functions do not have a definition available, unlike component or custom functions. They are handled by the PHP engine, and documented.
For example, strlen is a PHP native function, which measures the length of a string, in bytes. There are a lot of PHP functions available.
PHP native functions may be disabled at startup, with the disabled_functions directive.
It is possible to reach PHP native function definitions on the php.net website by adding their name in the URL. For example: https://www.php.net/strlen.
<?php
print_r(get_defined_functions());
?>