Generate Random number for WordPress for Human Check

I have a Custom Form and wanted to secure the Form submission with Human Check, by asking visitors to enter the given Random Number to validate.

Below is the function to generate a Random number:

function random_number() {
    $rand1 = rand(10, 100);
    $rand2 = rand(10, 100);
    $randnum = $rand1 * $rand2;
return $randnum;
}

Below is the Validation Form the Human Check:

<label for="humanCheck">
    <?php echo random_number(); ?>
</label>
<input id="humanCheck" type="number" name="message_human" placeholder="Enter above number">

Below is the Validation Code to check whether visitor entered the given random number:

$randomnumbercheck = echo random_number();
if (!$human == 0) {
if ($human != $randomnumbercheck) {
    response("error", $not_human); //not human!
} else {
/// form submission
}

Problem is random numbers generated by the random_number() function in both Validation form & validation code are different and it is always a false validation.