You can use the code below to implement a simple captcha using gd library of PHP. As you are beginner, here is a sample code for quick testing, it covers font-sizing also:
<?php
session_start();
header('Content-type: image/jpeg');
$text = rand(1000, 9999);
$font_size = 30;
$image_width = 200;
$image_height = 40;
$image = imagecreate($image_width, $image_height);
imagecolorallocate($image, 255, 255, 255);
$text_color = imagecolorallocate($image, 0, 0, 0);
for ($x=1; $x<=40; $x++) {
$x1 = rand(1, 100);
$y1 = rand(1, 100);
$x2 = rand(1, 100);
$y2 = rand(1, 100);
imageline($image, $x1, $y1, $x2, $y2, $text_color);
}
imagettftext($image, $font_size, 0, 15, 30, $text_color, 'FREESCPT.ttf', $text);
imagejpeg($image);
?>
In imagettftext function:
imagettftext($image, $font_size, $angle, $x, $y, $text_color, '$font-family', $text);
$image is the $imagecreate function
$font_size is the size of font you want.
$angle is the angle of the fonts tilted
$x and $y are coordinates.
$text_color is the imagecolorallocate function
$font-family is the family of font you want to use
$text is the text or random text to be displayed
Here is the good tutorial on how to build captcha in php -> link