I noticed that PHP’s internal functions never use strings for pre-defined or limited values, only constants.

For example:

pad_type:

Optional argument pad_type can be STR_PAD_RIGHT, STR_PAD_LEFT, or STR_PAD_BOTH. If pad_type is not specified it is assumed to be STR_PAD_RIGHT.

What’s the reason for not using a string as parameter here?

str_pad($test, 10, 0, 'left') seems a lot simpler than str_pad( $test, 10, 0, STR_PAD_LEFT)

(This is more of a meta question. I hope it’s OK to ask here.)