Generating a PNG CAPTCHA Image with PHP

CAPTCHA images are increasingly used in web forms, and implementing them with PHP can be more convenient than using JavaScript.

This article presents a basic PHP script that generates a PNG CAPTCHA, outlining each step: creating an image canvas, setting a random background color, defining font color and style, generating a four‑character random code, drawing each character with random rotation and position, adding noise lines and dots to hinder automated recognition, outputting the PNG image, and finally releasing the image memory.

The script can be wrapped into a reusable function for easier integration into future projects.

<?php
session_start();
header('Content-type: image/png');
// Create image
$im = imagecreate($x=130,$y=45);
// Allocate background color (first call fills background)
$bg = imagecolorallocate($im,rand(50,200),rand(0,155),rand(0,155));
// Font color
$fontColor = imageColorAllocate($im,255,255,255);
// Font style (place the .ttf file in the same directory as this script)
$fontstyle = 'rock.ttf';
// Generate random characters
for($i = 0; $i < 4; $i++) {
    $randAsciiNumArray = array(rand(48,57),rand(65,90));
    $randAsciiNum = $randAsciiNumArray[rand(0,1)];
    $randStr = chr($randAsciiNum);
    imagettftext($im,30,rand(0,20)-rand(0,25),5+$i*30,rand(30,35),$fontColor,$fontstyle,$randStr);
    $authcode .= $randStr;
}
// Store the code in session for later verification
$_SESSION['authcode'] = $authcode;
// Add interference lines
for($i=0;$i<8;$i++){
    $lineColor = imagecolorallocate($im,rand(0,255),rand(0,255),rand(0,255));
    imageline($im,rand(0,$x),0,rand(0,$x),$y,$lineColor);
}
// Add interference dots
for($i=0;$i<250;$i++){
    imagesetpixel($im,rand(0,$x),rand(0,$y),$fontColor);
}
// Output image
imagepng($im);
// Free memory
imagedestroy($im);
?>