Check if a variable is a natural number

If you don't require decimal points: ctype_digit or filter_var($var, FILTER_VALIDATE_INT).
If you do: filter_var($var, FILTER_VALIDATE_FLOAT).

ctype_digit does what you want:

Checks if all of the characters in the provided string, text, are numerical.

(Before PHP 5.1.0, this function returned TRUE when text was an empty string.)