PHP Random Text to Image (Captcha Generator)

<?php

function Generate() {

    // Set the content-type to display the image properly.
    header('Content-type: image/png');

    // Create the image. imagecreatetruecolor(Width, Height);
    $image = imagecreatetruecolor(400, 30);

    // Set the colors to be used. These use RGB color codes.
    // View the following link to easliy find RGB color codes:
    // http://www.color-he*****m/
    $white = imagecolorallocate($image, 255, 255, 255); // Used for the background fill, see below. (Default White = 255, 255, 255)
    $text_shadow = imagecolorallocate($image, 128, 128, 128); // Used for the text shadowing, see below. (Default Gray = 128, 128, 128)
    $text_color = imagecolorallocate($image, 0, 0, 0); // Used for the text color, see below. (Default Black = 0, 0, 0)
    
    imagefilledrectangle($image, 0, 0, 399, 29, $white); // Fills the background in white.

    // Call for the random text to be used and set the value.
    $text = RandomText();

    // You may change this to your own .ttf font file.
    // Thanks and credit to the following website and author for this great default font!
    // http://www.dafon*****m/chaos-times.font
    $font = 'ChaosTimes_lig.ttf';

    // Adds gray shadowing to the text, using the $text_shadow color.
    // This also adds slightly more distortion to the image, making it
    // slightly more difficult.
    imagettftext($image, 20, 0, 11, 21, $text_shadow, $font, $text);

    // Random text and colors have been set, time to put it all together.
    // This is also where we use $text_color to set the font color.
    imagettftext($image, 20, 0, 10, 20, $text_color, $font, $text);

    // Using imagepng() results in clear text compared to imagejpeg().
    // If you wanted to distort the image even more, you could use
    // imagejpeg($im); in place of the following.  Be sure to update
    // header('Content-type: image/png'); TO header('Content-type: image/jpeg');
    imagepng($image);
    imagedestroy($image);
}

// Randomly generates the text to be shown.
// The following line is also where to edit the different values,
// such as the $length which is how long the captcha will be.
// Also $chars which contains all the things to use in the captcha.
function RandomText($length = 8, $chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ") {
    $charl = strlen($chars); // Here we get the length of $chars, used to create random text from it.
    $string = ''; // Make sure the string is empty. Shouldn't normally be required but a good habit.
    
    for ($i=0; $i<$length; $i++) { // If you require some explaining here please view: https://goo .gl/yYjC2j
        $string .= $chars[rand(0, $charl - 1)]; // Set $string to random text using $chars as the text options.
    }
    return $string; // Return the random text generated.
}
// Output the finished image
echo Generate();
?>