Where can one find the source code for predefined PHP functions?

To find the source code for predefined PHP functions, you can visit the PHP manual website (https://www.php.net/manual/en/) and search for the specific function you are interested in. The source code for each predefined PHP function is typically available on the respective function's documentation page.

// Example: Finding the source code for the strlen function
echo file_get_contents('https://www.php.net/manual/en/function.strlen.php');