如何编写PHP验证码函数实现高效验证？

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phpfunction code() { $im=imagecreatetruecolor(100, 40); $black=imagecolorallocate($im, 0, 0, 0); $white=imagecolorallocate($im, 255, 255, 255); imagefill($im, 0, 0, $white); $strarr=array_merge(range(0, 9), range('a', 'z'), range('A', 'Z')); shuffle($strarr);}

<?php function code(){ $im = imagecreatetruecolor(100, 40); $black = imagecolorallocate($im, 0, 0, 0); $white = imagecolorallocate($im, 255, 255, 255); imagefill($im,0,0,$white); $strarr=array_merge(range(0,9),range(a,z),range(A,Z)); shuffle($strarr); $str=join(array_slice($strarr,0,4)); $mm=rand(0,40); $aaa=rand(0,0); $bbb=rand(0,255); $ccc=rand(0,255); $color1=imagecolorallocate($im,$aaa,$bbb,$ccc); imagestring($im,5,$mm,10,$str,$color1); for($i=0;$i<700;$i++){ $aa=rand(0,255); $bb=rand(0,255); $cc=rand(0,255); $color=imagecolorallocate($im,$aa,$bb,$cc); $a=rand(0,100); $b=rand(0,40); imagesetpixel($im,$a,$b,$color); } for($t=0;$t<3;$t++){ $a1=rand(0,255); $b1=rand(0,255); $c1=rand(0,255); $color2=imagecolorallocate($im,$a1,$b1,$c1); $a2=rand(0,100); $b2=rand(0,40); $a3=rand(0,100); $b3=rand(0,40); imageline($im,$a2,$b2,$b3,$b3,$color2); } header("content-type:image/png"); imagepng($im); } code() ; ?>