For the sake of completeness, here is an example for imagechar. 
The base-image automatically adjusts to the size and the height of the given string. Using the rand()-function the y-position of each char is slightly varied with every loop-run. You can easily rewrite the script to use a randomly generated string - the one given here just serves as an example.

<?php

$string = '1 2 3 4 5 6 7 8 9 A B C D E F G';
$font_size = 5;
$width=imagefontwidth($font_size)*strlen($string);
$height=imagefontheight($font_size)*2;
$img = imagecreate($width,$height);
$bg = imagecolorallocate($img,225,225,225);
$black = imagecolorallocate($img,0,0,0);
$len=strlen($string);

for($i=0;$i<$len;$i++)
{
    $xpos=$i*imagefontwidth($font_size);
    $ypos=rand(0,imagefontheight($font_size));
    imagechar($img,$font_size,$xpos,$ypos,$string,$black);
    $string = substr($string,1);    
    
}
header("Content-Type: image/gif");
imagegif($img); 
imagedestroy($img);
?>

A quick function to automatically generate a multi line image from a string, with the image size automatically calculated from the string itself.

<?php

function multilineimage($string){

    // Probably not the best way of handling newlines, but bar OS9, doesn't really cause a problem
    $string = str_replace("\r","",$string);
    $string = explode("\n",$string);

    $maxlen = 0;
    foreach ($string as $str){
        if (strlen($str) > $maxlen){
            $maxlen = strlen($str);
        }
    }

    // Set font size
    $font_size = 4;

    // Create image width dependant on width of the string
    $width  = imagefontwidth($font_size)*$maxlen;
    // Set height to that of the font
    $height = imagefontheight($font_size) * count($string);
    // Create the image pallette
    $img = imagecreate($width,$height);
    // Grey background
    $bg    = imagecolorallocate($img, 205, 255, 255);
    // White font color
    $color = imagecolorallocate($img, 0, 0, 0);

    $ypos = 0;

    foreach ($string as $str){

        $len = strlen($str);
        for($i=0;$i<$len;$i++){
            // Position of the character horizontally
            $xpos = $i * imagefontwidth($font_size);
            // Draw character
            imagechar($img, $font_size, $xpos, $ypos, $str, $color);
            // Remove character from string
            $str = substr($str, 1);   
        }
        $ypos = $ypos + imagefontheight($font_size);   
    }

    // Return the image 
    header("Content-Type: image/gif");
    imagegif($img);
    // Remove image
    imagedestroy($img);
}

multilineimage("This is an image
This is line 2\nLine 3
Line 4");

?>