Built-in functions refer to functions supported by PHP by default. PHP has many standard and commonly used processing functions built-in, including string processing, array functions, file processing, session and cookie processing, etc.

Let’s take the string processing function as an example. String replacement can be achieved through the built-in function str_replace. The following example replaces "jobs" with "steven jobs":

<?php

$str = 'i am jobs.';
$str = str_replace('jobs ', 'steven jobs', $str);
echo $str; //The result is "i am steven jobs"

?>

Also Some functions are supported through other extensions, such as mysql database processing functions, GD image processing functions, mail processing functions, etc. PHP loads some commonly used extension libraries by default. We can install or load other extension libraries to increase PHP's processing functions. .