Change Locations of each letter in home made captcha - location

Question

So my home made captcha displays the text in a straight line but I think because of this, the bots can still sign up - they're getting in.
Can someone show me how to revise my code below to add a function or something that would make each letter display a little higher or lower than the other letters? Please don't mind the dirty coding as I'm not a PHP professional. Thank you everyone for your help.
$image = imagecreatetruecolor(70, 20);
for ($i=0; $i < rand(20,40); $i++) {
$x = rand(0, 70);
$y = rand(0, 20);
imageline($image, $x, $y, $x+rand(0,10), $y+rand(0,240), imagecolorallocate($image, rand(0,255),rand(0,190),rand(1,90)));
imageline($image, $x, $y, $x+rand(0,11), $y+rand(0,34), imagecolorallocate($image, 255,rand(50,240),rand(241,240)));
}
$s = rand(1, 240);
$x = rand(50, 240);
$f = rand(1, 4);
$d = rand(1, 1);
$c = rand(1, 4);
imagestring($image, $c, 3, $f, $_GET["T"], imagecolorallocate($image, $s,$x,$s));
imagestring($image, $c, 4, $f, $_GET["T"], imagecolorallocate($image, 255,rand(50,240),rand(241,240)));
imagecolortransparent($image, imagecolorallocate($image, 255, 0, 0));
imageinterlace($image);
header("Content-type: image/gif");
imagegif($image);
imagedestroy($image);

Answer 1

Le code...
$image = imagecreatetruecolor(70, 20);
for ($i=0; $i < rand(20,40); $i++) {
$x = rand(0, 70);
$y = rand(0, 20);
imageline($image, $x, $y, $x+rand(0,10), $y+rand(0,240), imagecolorallocate($image, rand(0,255),rand(0,190),rand(1,90)));
imageline($image, $x, $y, $x+rand(0,11), $y+rand(0,34), imagecolorallocate($image, 255,rand(50,240),rand(241,240)));
}
$s = rand(1, 240);
$x = rand(50, 240);
$f = rand(1, 4);
$d = rand(1, 1);
$pos_x = rand(5,10);
$strArr = str_split($_GET["T"]);
foreach ($strArr as $str){
$font_size = rand(1, 5); // range from 1-5
imagestring($image, $font_size, $pos_x, $f, $str, imagecolorallocate($image, $s,$x,$s));
imagestring($image, $font_size, $pos_x, $f, $str, imagecolorallocate($image, 255,rand(50,240),rand(241,240)));
$pos_x = $pos_x + rand(20,30); //adjust your letter spaceing, depend on max char per chaptcha
}
imagecolortransparent($image, imagecolorallocate($image, 255, 0, 0));
imageinterlace($image);
header("Content-type: image/gif");
imagegif($image);
imagedestroy($image);
nothing special, but functional by your requirements :)
added rand size and rand letter space :)