Ti suggerisco di iniziare leggendo attentamente il manuale ufficiale:

http://www.php.net/manual/en/languag...e-juggling.php
http://www.php.net/is_float


is_float (PHP 4, PHP 5)
is_float — Finds whether the type of a variable is float

Note: To test if a variable is a number or a numeric string (such as form input, which is always a string), you must use is_numeric().