<?php
session_start();
echo $_SESSION['secure'] = mt_rand(1000,9999);

if(isset($_POST['check_captcha'])){
    if($_SESSION['secure'] == $_POST['captcha']){
    echo"match";
    }else{
    echo " No match";
    }
}
?>


<form method="POST" action="<?php echo $_SERVER['PHP_SELF'];?>">
    <input type="text" name="captcha"/>
    <input type="submit" name="check_captcha" value="login"/>
</form>