All basic PHP functions which i tried returned unexpected results. I would just like to check whether some variable only contains numbers. For example: when i spread my script to the public i cannot require users to only use numbers as string or as integer. For those situation i wrote my own function which handles all inconveniences of other functions and which is not depending on regular expressions. Some people strongly believe that regular functions slow down your script.

The reason to write this function:
1. is_numeric() accepts values like: +0123.45e6 (but you would expect it would not)
2. is_int() does not accept HTML form fields (like: 123) because they are treated as strings (like: "123").
3. ctype_digit() excepts all numbers to be strings (like: "123") and does not validate real integers (like: 123).
4. Probably some functions would parse a boolean (like: true or false) as 0 or 1 and validate it in that manner.

My function only accepts numbers regardless whether they are in string or in integer format.
<?php
    /**
     * Check input for existing only of digits (numbers)
     * @author Tim Boormans <[email protected]>
     * @param $digit
     * @return bool
     */
    function is_digit($digit) {
        if(is_int($digit)) {
            return true;
        } elseif(is_string($digit)) {
            return ctype_digit($digit);
        } else {
            // booleans, floats and others
            return false;
        }
    }
?>

I just wanted to clarify a flaw in the function is_digit() suggested by "info at directwebsolutions dot nl " .. 
It returns true in case of negative integers and false in case of strings that contain negative integers .
 example:
is_digit(-10); // returns ture
is_digit('-10'); // returns false

Note that an empty string is also false:
ctype_digit("") // false

Please note that ctype_digit() will say true for strings such as '00001', which are not technically valid representations of integers, while saying false to strings such as '-1', which are. It's basically a faster version of the regex /^\d+$/. As the name says, it answers the question "does this string contain only digits" literally. It does not answer "is this a valid representation of an integer". If that's what you want, use is_int(filter_var($val, FILTER_VALIDATE_INT)) instead.

Interesting to note that you must pass a STRING to this function, other values won't be typecasted (I figured it would even though above explicitly says string $text).

I.E.

<?PHP
$val = 42; //Answer to life
$x = ctype_digit($val);
?>

Will return false, even though, when typecasted to string, it would be true.

<?PHP
$val = '42';
$x = ctype_digit($val);
?>

Returns True.

Could do this too:

<?PHP
$val = 42;
$x = ctype_digit((string) $val);
?>

Which will also return true, as it should.

ctype_digit() will treat all passed integers below 256 as character-codes. It returns true for 48 through 57 (ASCII '0'-'9') and false for the rest.

ctype_digit(5) -> false
ctype_digit(48) -> true
ctype_digit(255) -> false
ctype_digit(256) -> true

(Note: the PHP type must be an int; if you pass strings it works as expected)

Also note that

<?php ctype_digit("-1");   //false ?>

is_numeric gives true by f. ex. 1e3 or 0xf5 too. So it's not the same as ctype_digit, which just gives true when only values from 0 to 9 are entered.

Using is_numeric function is quite faster than ctype_digit.

is_numeric took 0.237 Seconds for one million runs. while ctype_digit took 0.470 Seconds.