<?php
 
  function warpedCaptcha($text)
  {
   /*
    this function takes the original captcha text and shuffles
	the vowels + y around in order to deliver more variable 
	captcha text
   */
   if (5 > rand(0,6)) return $text;
   //only parttime warping
   $ltrs = array('b'=>'b','c'=>'c','d'=>'d','f'=>'f','g'=>'g','h'=>'h','j'=>'j',
               'k'=>'k','l'=>'l','m'=>'m','n'=>'n','p'=>'p','q'=>'q','r'=>'r',
			   's'=>'s','t'=>'t','v'=>'v','w'=>'w','x'=>'x','z'=>'z');

  $vkeys = $vltrs = explode(',','a,e,i,o,u,y');
  shuffle($vkeys);
  $vowels = array();
  foreach($vkeys as $ndx=>$vkey) $vowels[$vkey] = $vltrs[$ndx];
  //the vowels + y are now jumbled up
 
  $ltrs = array_merge($ltrs,$vowels);
  //full associative array of alphabet with randomized vowels + y
  $text = str_split($text);
 
  $captcha = '';
  foreach($text as $txt) $captcha .= $ltrs[$txt];
  return $captcha;
 }
 
 /*
  xsixlw.txt is a long string of 6 letter words concatenated together.
  We pick a word at random from this text
 */
 $fp = fopen('xsixlw.txt',"r");
 $count = filesize('xsixlw.txt')/6 - 1;
 $pos = 6*rand(0,$count);
 fseek($fp,$pos);
 $captcha = warpedCaptcha(trim(fread($fp,6)));
 fclose($fp);
 		  
 
 
 session_start();
 $_SESSION['captcha'] = $captcha;
 //store the captcha to check later once the user has solved it
 session_write_close();
 
 $im = imagecreatetruecolor(200,70);//200 x 70 pixel image
 $black = imagecolorallocate($im,0,0,0);
 imagecolortransparent($im,$black);//give it a black background

 switch(rand(0,4))
 {
  case 0:$color = imagecolorallocate($im,34,155,91);break;
  case 1:$color = imagecolorallocate($im,233,26,74);break;
  case 2:$color = imagecolorallocate($im,233,26,195);break;
  case 3:$color = imagecolorallocate($im,244,178,19);break;
  case 4:$color = imagecolorallocate($im,53,125,199);break;
 } 
 //pick a random color for the text

 $x = 20;$y = 47;//the starting position for drawing
 for ($i=0;$i<6;$i++)
 {
  $angle = rand(-8,8) + rand(0,9)/10;
  $fontsize = rand(22,32);//pick a random font size
  $letter = substr($captcha,$i,1);
  $coords = imagettftext($im,$fontsize,$angle,$x,$y,$color,'oldsans.ttf',$letter);
  //draw each letter
  $x += ($coords[2]-$x) + 1;
 }


header('Content-type:image/png');
imagepng($im);
imagedestroy($im);
?>

http://jreply.com/freebies/simplephpcaptcha.html

CAPTCHAs are often too hard to solve - hard enough to annoy real human users.  This CAPTCHA is easy to understand and to modify and is strong enough to keep out bots whilst being easy on human eyes.  

The source code below will not work on its own - it requires a TTF font file and a word list both of which are available in the ZIP archive available for download from the URL given here.

Simple PHP CAPTCHA

PHP