How to validate my PHP captcha case insensitive? - php

Question

I would like users submitting a captcha to be able to enter a mix of uppercase and lowercase, regardless of what the captcha image says, and the captcha to still be validated.
So for example, if the captcha image says 'r9zvk1', users should be able to input 'R9zvk1' and still have the captcha validated as correct.
My current captcha code:
<?php
session_start();
$permitted_chars = 'abcdefghjklmnpqrstuvwxyz0123456789';
function generate_string($input, $strength = 10) {
$input_length = strlen($input);
$random_string = '';
for($i = 0; $i < $strength; $i++) {
$random_character = $input[mt_rand(0, $input_length - 1)];
$random_string .= $random_character;
}
return $random_string;
}
$image = imagecreatetruecolor(200, 50);
imageantialias($image, true);
$colors = [];
$red = rand(125, 175);
$green = rand(125, 175);
$blue = rand(125, 175);
for($i = 0; $i < 5; $i++) {
$colors[] = imagecolorallocate($image, $red - 20*$i, $green - 20*$i, $blue - 20*$i);
}
imagefill($image, 0, 0, $colors[0]);
for($i = 0; $i < 10; $i++) {
imagesetthickness($image, rand(2, 10));
$line_color = $colors[rand(1, 4)];
imagerectangle($image, rand(-10, 190), rand(-10, 10), rand(-10, 190), rand(40, 60), $line_color);
}
$black = imagecolorallocate($image, 0, 0, 0);
$white = imagecolorallocate($image, 255, 255, 255);
$textcolors = [$black, $white];
$fonts = [dirname(__FILE__).'/fonts/font.ttf'];
$string_length = 5;
$captcha_string = generate_string($permitted_chars, $string_length);
$_SESSION['captcha_text'] = $captcha_string;
for($i = 0; $i < $string_length; $i++) {
$letter_space = 170/$string_length;
$initial = 15;
imagettftext($image, 24, rand(-15, 15), $initial + $i*$letter_space, rand(25, 45), $textcolors[rand(0, 1)], $fonts[array_rand($fonts)], $captcha_string[$i]);
}
header('Content-type: image/png');
imagepng($image);
imagedestroy($image);
?>
My current captcha validation code:
<?PHP
session_start();
if($_POST['captcha'] != $_SESSION['captcha_text']) die("Sorry, the CAPTCHA code entered was incorrect!");
session_destroy();
?>