One more question about CAPTCHA

[slpctrl]

Alright, here's my code to produce my image for the captcha:

 

<?php
session_start();
$str1 = md5(microtime() * mktime());
$str = substr($str1,0,5);
$captcha = imagecreatefrompng("./captcha.png");
$black = imagecolorallocate($captcha,0,0,0);
$line = imagecolorallocate($captcha,rand(0,250),rand(0,239),rand(0,240));
imageline($captcha,rand(0,50),rand(0,100),rand(0,200),rand(0,200),$line);
imageline($captcha,rand(0,50),rand(0,100),rand(0,200),rand(0,200),$line);
imageline($captcha,rand(0,50),rand(0,100),rand(0,200),rand(0,200),$line);
imageline($captcha,rand(0,50),rand(0,100),rand(0,200),rand(0,200),$line);
imageline($captcha,rand(0,50),rand(0,100),rand(0,200),rand(0,200),$line);
imageline($captcha,rand(0,50),rand(0,100),rand(0,200),rand(0,200),$line);
imageline($captcha,rand(0,50),rand(0,100),rand(0,200),rand(0,200),$line);
imageline($captcha,rand(0,50),rand(0,100),rand(0,200),rand(0,200),$line);
imageline($captcha,rand(0,50),rand(0,100),rand(0,200),rand(0,200),$line);
imageline($captcha,rand(0,50),rand(0,100),rand(0,200),rand(0,200),$line);
imageline($captcha,rand(0,50),rand(0,100),rand(0,200),rand(0,200),$line);
imageline($captcha,rand(0,50),rand(0,100),rand(0,200),rand(0,200),$line);
imagestring($captcha,rand(1,5),rand(0,100),rand(0,50),$str,$black);
$_SESSION['str'] = md5($str);
header("Content-type: image/png");
imagepng($captcha);
?>

 

But now, how would I check if the text submitted matches the text on the captcha? I know this question might seem dumb I donno, I haven't done any PHP in a while (been learning lots of python lately ), and can't get my head around how to check. Any help is appreciated thanks.

[BlueSkyIS]

when the user submits their guess, see if the md5() of it matches the value of $_SESSION['str']