<"#FFFFFF";  //背景色

	var $TFonts = array("font.ttf");
	var $TFontSize=array(17,20); //字体大小范围
	var $TFontAngle=array(-20,20); //旋转角度

	var $Chars  = "0123456789";     //验证码范围（字母数字）

	var $Code  = array();       //验证码
	var $Image  = "";       //图形对象
	var $FontColors=array('#f36161','#6bc146','#5368bd'); //字体颜色,红绿蓝
	var $TPadden = 0.75;///字符间距,多少个字符
	var $Txbase = 5;///x轴两边距离
	var $Tybase =5 ;///y轴两边距离
	var $TLine =true; ///画干扰线


	public function RandRSI() ///生成验证码
	{
		$this->TFontAngle=range($this->TFontAngle[0],$this->TFontAngle[1]);
		$this->TFontSize=range($this->TFontSize[0],$this->TFontSize[1]);

		$arr=array();
		$Chars=$this->Chars;
		$TFontAngle=$this->TFontAngle;
		$TFontSize=$this->TFontSize;
		$FontColors=$this->FontColors;
		$code="";
		$font=dirname(__FILE__)."/font/".$this->TFonts[0];

		$charlen=strlen($Chars)-1;
		$anglelen=count($TFontAngle)-1; // 角度范围
		$fontsizelen=count($TFontSize)-1; // 角度范围
		$fontcolorlen=count($FontColors)-1; // 角度范围

		for($i=0;$i<$this->Length;$i++) ///得到字符与颜色
		{
			$char=$Chars[rand(0,$charlen)]; ///得到字符
			$angle=$TFontAngle[rand(0,$anglelen)]; ///旋转角度
			$fontsize=$TFontSize[rand(0,$fontsizelen)]; ///字体大小
			$fontcolor=$FontColors[rand(0,$fontcolorlen)]; ///字体大小

			$bound=$this->_calculateTextBox($fontsize,$angle,$font,$char); ///得到范围

			$arr[]=array($fontsize,$angle,$fontcolor,$char,$font,$bound); ///得到矩形框
			$code.=$char;
		}
		$this->Code=$arr; //验证码
		return $code;
	}

	public function Draw() ///画图
	{
		if(empty($this->Code)) $this->RandRSI();
		$codes=$this->Code; ///用户验证码


		$wh=$this->_getImageWH($codes);

		$width=$wh[0];
		$height=$wh[1]; ///高度

		$this->Width=$width;
		$this->Height=$height;

		$this->Image = imageCreate( $width, $height );
		$image=$this->Image;

		$back = $this->_getColor2($this->_getColor( $this->BgColor)); ///背景颜色
		imageFilledRectangle($image, 0, 0, $width, $height, $back); ///填充背景

		$TPadden=$this->TPadden;

		$basex=$this->Txbase;
		$color=null;
		foreach ($codes as $v) ///逐个画字符
		{
			$bound=$v[5];
			$color=$this->_getColor2($this->_getColor($v[2]));
			imagettftext($image, $v[0], $v[1], $basex, $bound['height'],$color , $v[4], $v[3]);
			$basex=$basex+$bound['width']*$TPadden-$bound['left'];///计算下一个左边距
		}
		$this->TLine"Content-type: image/png");
		imagepng( $image);
		imagedestroy($image);

	}

	/**
	 *通过字体角度得到字体矩形宽度*
	 *
	 * @param int $font_size 字体尺寸
	 * @param float $font_angle 旋转角度
	 * @param string $font_file 字体文件路径
	 * @param string $text 写入字符
	 * @return array 返回长宽高
	 */
	private function _calculateTextBox($font_size, $font_angle, $font_file, $text) {
		$box = imagettfbbox($font_size, $font_angle, $font_file, $text);

		$min_x = min(array($box[0], $box[2], $box[4], $box[6]));
		$max_x = max(array($box[0], $box[2], $box[4], $box[6]));
		$min_y = min(array($box[1], $box[3], $box[5], $box[7]));
		$max_y = max(array($box[1], $box[3], $box[5], $box[7]));

		return array(
		'left' => ($min_x >= -1) "htmlcode">

$rsi = new Utils_Caption();
$rsi->TFontSize=array(15,17);
$rsi->Width=50;
$rsi->Height=25;
$code = $rsi->RandRSI();
session_start();
$_SESSION["CHECKCODE"] = $code;
$rsi->Draw();


运行效果：
 
点击下载源码
大家可以修改代码中的相应数值达到自己需要的复杂度