Captcha code validation not working - php

Question

I have the following code validation php script:
if(empty($_POST['captcha_code'])) {
$error = 1;
$code[3] = 'color:#FF0000;';
} else {
include_once "formfiles/captcha.php";
$randomnr = new Securimage();
$valid = $randomnr->check($_POST['captcha_code']);
if(!$valid) {
$error = 1;
$code[3] = 'color:#FF0000;';
$code[4] = '<strong><span style="color:#FF0000;">Incorrect code</span></strong>';
}
}
and this captcha php code:
<?php
Securimage();
exit();
function Securimage()
{
$randomnr = rand(1000, 9999);
$_SESSION['randomnr2'] = md5($randomnr);
$im = imagecreatetruecolor(100, 38);
$white = imagecolorallocate($im, 255, 255, 255);
$grey = imagecolorallocate($im, 150, 150, 150);
$black = imagecolorallocate($im, 0, 0, 0);
imagefilledrectangle($im, 0, 0, 200, 35, $black);
//path to font - this is just an example you can use any font you like:
$font = dirName(__FILE__).'/calvin.ttf';
imagettftext($im, 20, 4, 22, 30, $grey, $font, $randomnr);
imagettftext($im, 20, 4, 15, 32, $white, $font, $randomnr);
imagegif($im);
imagedestroy($im);
}
?>
After submitting my form I always get an awkward bit of code starting with gif. Where is my fault? Can somebody please help me?

Answer 1

You need to decalare header about the content type to browser recognize it as image. Try this
header('Content-Type: image/gif');
Securimage();
exit();