How to Reliably Check if a Variable Represents an Integer in PHP?

Checking if a Variable is an Integer in PHP

When working with user input, it's crucial to validate the data to ensure its integrity. One common validation task is checking if a variable represents an integer. PHP provides the is_int() function for this purpose, but it may not always produce the desired results.

Why is_int() Might Not Work as Expected

Using is_int() may return false positives for values that appear to be integers but aren't recognized as such by the function. For example, consider the case where a user enters "1" as a page number:

<code class="php">$page = $_GET['p'];</code>

In this case, $page will be assigned the string "1," which will result in is_int($page) returning false.

Alternative Validation Methods

To reliably check if a variable is an integer, consider using alternative methods that provide more accurate results:

1. FILTER_VALIDATE_INT Way

The FILTER_VALIDATE_INT filter can be used to validate integer inputs:

<code class="php">if (filter_var($page, FILTER_VALIDATE_INT) === false) {
    // Not an integer
}</code>

Output:

TEST -1: -1 (type:integer) Your variable is an integer ✔
TEST 0 : 0 (type:integer) is an integer ✔
TEST 1 : 42 (type:integer) is an integer ✔
TEST 2 : 4.2 (type:double) is not an integer ✘
TEST 3 : 0.42 (type:double) is not an integer ✘
TEST 4 : 42 (type:double) is an integer ✔
TEST 5 : 42 (type:string) is an integer ✔
TEST 6 : a42 (type:string) is not an integer ✘
TEST 7 : 42a (type:string) is not an integer ✘
TEST 8 : 36 (type:integer) is an integer ✔
TEST 9 : 1337 (type:double) is an integer ✔

2. Casting Comparison Way

Comparing the string representation of a variable with its integer representation can also reveal whether it's an integer:

<code class="php">if (strval($page) !== strval(intval($page))) {
    // Not an integer
}</code>

Output:

TEST -1: -1 (type:integer) Your variable is an integer ✔
TEST 0 : 0 (type:integer) is an integer ✔
TEST 1 : 42 (type:integer) is an integer ✔
TEST 2 : 4.2 (type:double) is not an integer ✘
TEST 3 : 0.42 (type:double) is not an integer ✘
TEST 4 : 42 (type:double) is an integer ✔
TEST 5 : 42 (type:string) is an integer ✔
TEST 6 : a42 (type:string) is not an integer ✘
TEST 7 : 42a (type:string) is not an integer ✘
TEST 8 : 36 (type:integer) is an integer ✔
TEST 9 : 1337 (type:double) is an integer ✔

3. CTYPE_DIGIT Way

Using the ctype_digit function, you can check for positive numbers and 0:

<code class="php">if (!ctype_digit(strval($page))) {
    // Not an integer
}</code>

Output:

TEST -1: -1 (type:integer) # Your variable is not an integer ✘
TEST 0 : 0 (type:integer) is an integer ✔
TEST 1 : 42 (type:integer) is an integer ✔
TEST 2 : 4.2 (type:double) is not an integer ✘
TEST 3 : 0.42 (type:double) is not an integer ✘
TEST 4 : 42 (type:double) is an integer ✔
TEST 5 : 42 (type:string) is an integer ✔
TEST 6 : a42 (type:string) is not an integer ✘
TEST 7 : 42a (type:string) is not an integer ✘
TEST 8 : 36 (type:integer) is an integer ✔
TEST 9 : 1337 (type:double) is an integer ✔

4. REGEX Way

Regular expressions can also be used to validate integers:

<code class="php">if (!preg_match('/^-?\d+$/', $page)) {
    // Not an integer
}</code>

Output:

TEST -1: -1 (type:integer) Your variable is an integer ✔
TEST 0 : 0 (type:integer) is an integer ✔
TEST 1 : 42 (type:integer) is an integer ✔
TEST 2 : 4.2 (type:double) is not an integer ✘
TEST 3 : 0.42 (type:double) is not an integer ✘
TEST 4 : 42 (type:double) is an integer ✔
TEST 5 : 42 (type:string) is an integer ✔
TEST 6 : a42 (type:string) is not an integer ✘
TEST 7 : 42a (type:string) is not an integer ✘
TEST 8 : 36 (type:integer) is an integer ✔
TEST 9 : 1337 (type:double) is an integer ✔

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