Php form validation numbers only

In this article, we will go over how to perform number, or numeric, validation of a form field filled in by a user, to make sure that the data entered is, in fact, a numeric value.

This may be necessary for a slew of forms filled out on the web. Examples of these are credit card numbers, zip codes, house numbers, telephone numbers, cash amounts, etc. These are all form fields where we want only numbers entered in to the form field. If any characters in the sequence are non-numeric, then we want to tell the user that the sequence of characters which they entered in are not valid. This is what is referred to as number validation.

Below is an example of a form field which checks for number validation. Only numbers can be entered in to the form field, only numeric values. If non-numeric values are entered, then the form field catches this and tells the user that s/he has made an error.

Result

Above is a form which checks for number validation. In this form, it is wanted that users enter numbers only into the form field. If a user enters a nonnumeric character, an error is thrown and the user is told to enter numbers only into the field.

PHP

So how do we perform number validation in PHP on a form field to make sure only numbers have been entered?

The answer is, we use a function in PHP called the is_numeric function.

The is_numeric function is used to check whether the character(s) which are entered. If all the characters are numeric, it returns a true value. If all the characters are not numeric, it returns a false, or not true, value.

Knowing now how this function works, we can implement it based on an if-else statement. If true, we can make it execute one statement. If false, we can make it execute another statement, such as the form above does.

HTML Code

This HTML code creates the text field in which a user enters characters.

The important thing we need to know from this HTML Code is the "name" attribute of the text field, which in this case is number. We will need to know this for the PHP code, because the "name" attribute is what we use in PHP of the HTML form field to test the characters that the user enters into this field.

PHP Code

Last week David Walsh posted about validating numeric values and digits with PHP on his excellent blog. I posted my own comments about his particular post but they never appeared so I decided to post a better response here.

The point of David’s post was that he needed to be able to validate between finding a numeric value (which might be a floating point number and therefore contain a decimal point) and whether a number only contains digits.

His solution was this:

/* numeric, decimal passes */
function validate_numeric($variable)
{
    return is_numeric($variable);
}

/* digits only, no dots */
function is_digits($element)
{
    return !preg_match ("/[^0-9]/", $element);
}

As I noted in my comment (which wasn’t published), I’m not sure why you’d wrap is_numeric() into his validate_numeric() function when all it’s doing is calling is_numeric(). Why not just call is_numeric() instead?

The is_digits() function on the other hand is actually pretty good. I initially thought there may be some issues with it but it works nicely but there is one simpler alternative.

is_integer – no good

In my comment I suggested that he could use the is_integer() function as long as the acceptable value was within the integer type range, and postive and negative values are acceptable. Of course, a digit only value might not be within the integer range, for example is_integer(12345678901234567890) would return false but does only contain numbers.

The problem with my initial assumption is that in PHP data submitted from a form that *does* only contain digits and *is* within the integer range will still return false from is_integer() because it will be considered to be a string. For example:

And then doing this when the form is submitted:

echo (int)is_integer((int)$_POST['foo']);

will print 0, meaning false. D’oh! Also an integer can contain – if it is a negative value so -123 will return true even though it contains more than just digits.

ctype_digit – not much better

The other alternative is the ctype_digit() function which saves having to mess around with regular expressions. ctype_digit checks if all of the characters in the passed in value are numerical. Only the numbers 0 to 9 are valid, and it doesn’t matter if the value is type cast as a string, which is what will happen from a form post.

However, ctype_digit it expects the input to be a string and will return false if the value passed in is an ineteger…

For example:

var_dump(ctype_digit(123));
var_dump(ctype_digit('123'));
var_dump(ctype_digit(12345678901234567890));
var_dump(ctype_digit('12345678901234567890'));

These display:

bool(false)
bool(true)
bool(false)
bool(true)

You should be able to use (string), strval() etc to cast the value as a string first and then pass it to ctype_digit but I’ve found if the number is too large it still doesn’t work.

For example, the first four below return true, but the last one returns false:

ctype_digit(strval(12345678901));
ctype_digit(strval(123456789012));
ctype_digit(strval(1234567890123));
ctype_digit(strval(12345678901234));
ctype_digit(strval(123456789012345));

Use the regexp

So my initial thought was just use is_integer() and someone else suggested to use ctype_digit but it would turn out that using the regular expression with preg_match is the best solution after all. Just remember when using regular expressions with PHP to use the preg* functions and not the ereg* functions because the latter are deprecated in PHP 6.

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