What is the best way to determine the PHP filename in which the source code is located?

To determine the PHP filename in which the source code is located, you can use the `__FILE__` magic constant in PHP. This constant returns the full path and filename of the current file. By using `__FILE__`, you can easily retrieve the filename of the PHP script being executed.

$filename = __FILE__;
echo "The current PHP filename is: " . $filename;