请教如何改验证里面字的大小

<?php<br /> <br />session_start();<br />//session_register('SafeCode');<br />$type = 'gif';<br />$width= 92;<br />$height= 30;<br />header("Content-type: image/".$type);<br />srand((double)microtime()*1000000);<br />$randval = randStr(4,"");<br />if($type!='gif'&&function_exists('imagecreatetruecolor')){<br />$im = @imagecreatetruecolor($width,$height);<br />}else{<br />$im = @imagecreate($width,$height);<br />}<br />$r = Array(225,211,255,223);<br />$g = Array(225,236,237,215);<br />$b = Array(225,236,166,125);<br />$key = rand(0,3);<br />$backColor = ImageColorAllocate($im,$r[$key],$g[$key],$b[$key]);<br />$borderColor = ImageColorAllocate($im,0,0,0);<br />$pointColor = ImageColorAllocate($im,255,170,255);<br />@imagefilledrectangle($im,0,0,$width -1,$height -1,$backColor);<br />@imagerectangle($im,0,0,$width-1,$height-1,"");<br />$stringColor = ImageColorAllocate($im,555,51,153);<br />for($i=0;$i<=100;$i++){<br />$pointX = rand(2,$width-2);<br />$pointY = rand(2,$height-2);<br />@imagesetpixel($im,$pointX,$pointY,$pointColor);<br />}<br />@imagestring($im,10,10,2,$randval,$stringColor);<br />$ImageFun='Image'.$type;<br />$ImageFun($im);<br />@ImageDestroy($im);<br />$_SESSION['validationcode'] = $randval;<br />function randStr($len=6,$format='NUMBER') {<br />switch($format) {<br />case 'ALL':<br />$chars='0123456789';break;<br />case 'CHAR':<br />$chars='';break;<br />case 'NUMBER':<br />$chars='0123456789';break;<br />default :<br />$chars='0123456789';<br />break;<br />}<br />$string="";<br />while(strlen($string)<$len)<br />$string.=substr($chars,(mt_rand()%strlen($chars)),1);<br />return $string;<br />}<br />;echo ' ';<br />?>