Use this code

<?php
$code=rand(1000,9999);
?>

<script>
    function myfun(){ 
        var x = document.getElementById('captcha1').value;
        var y = <?php echo $code  ?>;

        if (x == y) {
            return true;
        }
        else{
            return false;
        }
    }
<script>
<form>
<span-><?php echo $code;?></span>
<input id="captcha1" name="captcha" type="text">
<input id="button" type='submit'  onclick=" return myfun();" value='Submit'>
</form>