长尾词如何通过数字运算生成验证码？

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phpfunction getValidate($w, $h) { $img=imagecreate($w, $h); $gray=imagecolorallocate($img, 255, 255, 255); $black=imagecolorallocate($img, rand(0, 200), rand(0, 200), rand(0, 200)); $red=imagecolorallocate($img, 255, 0, 0);}

<?php getValidate(100,30); function getValidate($w,$h){ $img = imagecreate($w,$h); $gray = imagecolorallocate($img,255,255,255); $black = imagecolorallocate($img,rand(0,200),rand(0,200),rand(0,200)); $red = imagecolorallocate($img, 255, 0, 0); $white = imagecolorallocate($img, 255, 255, 255); $green = imagecolorallocate($img, 0, 255, 0); $blue = imagecolorallocate($img, 0, 0, 255); imagefilledrectangle($img, 0, 0, 100, 30, $black); for($i = 0;$i < 80;$i++){ imagesetpixel($img, rand(0,$w), rand(0,$h), $gray); } $num1 = rand(1,99); $num2 = rand(1,99); imagestring($img, 5, 5, rand(1,10), $num1, $red); imagestring($img,5,30,rand(1,10),getRand(), $white); imagestring($img,5,45,rand(1,10),$num2, $green); imagestring($img,5,65,rand(1,10),"=", $blue); imagestring($img,5,80,rand(1,10),"?", $red); header("content-type:image/png"); imagepng($img); imagedestroy($img); } function getRand(){ $code = rand(0,1); switch ($code) { case 0: return "+"; break; case 1: return "-"; break; default: # code... break; } } ?>