PHP: Negative Numbers and ctype_digit

Using ctype_digit(), you can only test whether the passed string is consisting of digits (0-9). The function does not check whether the string is holding a valid integer number that can possibly be negative.

However, you can use this regular expression and preg_match() for your purpose:

$teststr = "-1";

if (preg_match("/^-?[1-9][0-9]*$/", $teststr)) {
  echo 'String is a positive or negative number.';
}

This if statement with this regular expression ensures:

• that there can be a minus sign at the beginning of the string, but that must not be one
• that the number must start with a digit from 1 to 9, a 0 at the beginning is not allowed
• that this starting number can be followed by an arbitrary number of other digits from 0 to 9, but must not

So, with this code, you can identify negative as well as positive numbers of an arbitrary length.
2016-06-08 at 20:12