try filter_var function

filter_var($_POST['id'], FILTER_VALIDATE_INT);

use:

if(filter_var($_POST['id'], FILTER_VALIDATE_INT)) { 
    //Doing somethings...

}

In PHP $_POST values are always text (string type).

You can force a variable into the integer type like this:

$int_id = (int)$_POST['id'];

That will work if you are certain that $_POST['id'] should be an integer. But if you want to make absolutely sure that it contains only numbers from 0 to 9 and no other signs or symbols use:

if( ctype_digit( $_POST['id'] ) )
{
  $int_id = (int)$_POST['id'];
}

if you know it is a string variable (like post o get values), you can use:

function is_really_integer($var) {
  return $var == (string)(integer)$var;
}

Using is_numeric() for checking if a variable is an integer is a bad idea. This function will send TRUE for 3.14 for example. It's not the expected behavior

To do this correctly, you can use one of these options :

Considering this variables array :

$variables = [
    "TEST 0" => 0,
    "TEST 1" => 42,
    "TEST 2" => 4.2,
    "TEST 3" => .42,
    "TEST 4" => 42.,
    "TEST 5" => "42",
    "TEST 6" => "a42",
    "TEST 7" => "42a",
    "TEST 8" => 0x24,
    "TEST 9" => 1337e0
];

The first option (FILTER_VALIDATE_INT Way) :

# Check if your variable is an integer
if( ! filter_var($variable, FILTER_VALIDATE_INT) ){
  echo "Your variable is not an integer";
}

Output :

TEST 0 : 0 (type:integer) is not 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 ✔

The second option (CASTING COMPARISON Way) :

# Check if your variable is an integer
if ( strval($variable) != strval(intval($variable)) ) {
  echo "Your variable is not an integer";
}

Output :

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 ✔

The third option (CTYPE_DIGIT Way) :

# Check if your variable is an integer
if( ! ctype_digit(strval($variable)) ){
  echo "Your variable is not an integer";
}

Output :

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 ✔

The fourth option (REGEX Way) :

# Check if your variable is an integer
if( ! preg_match('/^\d+$/', $variable) ){
  echo "Your variable is not an integer";
}

Output :

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 ✔

Check it out: http://php.net/manual/en/function.ctype-digit.php - it validates if string contains only digits, so be sure not to pass an int to that function as it will most likely return false; However all values coming from $_POST are always strings so you are safe. Also it will not validate negative number such as -18 since - is not a digit, but you can always do ctype_digit(ltrim($number, '-'))

is_int checks the variable type which in your case is string; it would be the same as (integer)$v === $v as == does some real obscure things in order to compare two variables of a different type; you should always use === unless you want mess like "0af5gbd" == 0 to return true

Also, keep in mind that ctype_digit will not tell you if the string can be actually converted to a valid int since maximum integer value is PHP_INT_MAX; If your value is bigger than that, you will get PHP_INT_MAX anyway.

preg_match('/^\d+$/D',$variable) //edit 

The accepted answer using ctype_digit is correct, however you can make life easier using a function. This will covert the variable to a string, so you don't have to:

function is_num($x){
    if(!is_string($x)){
        $x=(string)$x;
    }
    if(ctype_digit($x)){
      return true;
    }
    return false;
}

Usage:

if (is_num(56)) {
    // its a number
}

if (is_num('56')) {
    // its a number
}

If you want to accept decimals too, use this:

function is_num($x){
    if(!is_string($x)){
        $x=(string)$x;
    }    
    if (strpos($x,'.')!==false) {      
        if(substr_count($x,'.')>1||strpos($x,'.')<1||strpos($x,'.')>=strlen($x)){
            return false;    
        }
        $x=str_replace('.','',$x);    
    }
    if(ctype_digit($x)){
        return true;
    }  
    return false;
}

Use filter_var but be careful it's return bool.

In case you have string '0' => you will get false

Right way to use:

if (filter_var($_POST['id'], FILTER_VALIDATE_INT) !== false) { 
    // Doing somethings...
}

Use the following, universal function to check all types:

function is_digit($mixed) {
    if(is_int($mixed)) {
        return true;
    } elseif(is_string($mixed)) {
        return ctype_digit($mixed);
    }

    return false;
}

I use:

is_int($val)||ctype_digit($val)

Note than this catch only positive integer strings

is_int is perfectly working there is not need of extra code

or you can check using is_numeric