Quickest way to check that a value in PHP only contains digits?

I could do this the painful way with a double开发者_运维技巧 for loop, but surely there's an inbuilt method? Will be used to validate phone number entries.

ctype_digit()

preg_match('/^\d+$/', $string);

Checks to see if the whole string is just digits.

If you want to match spaces as well:

preg_match('/^[0-9 ]+$/', $string);