Php verification code program

The code is as follows:

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Session_start (); $ Im = imagecreatetruecolor (100,30 ); // Assign color $ Bg = imagecolorallocate ($ im, 0, 0 ); $ Textcolor = imagecolorallocate ($ im, 255,255,255 ); // Mark the image For ($ I = 0; $ I <3; $ I ++ ){ $ Te1 = imagecolorallocate ($ instant, rand (0,255), rand (0,255), rand (0,255 )); Imageline ($ im, 0, rand (0, 15), 100, rand (0, 15), $ te1 ); } // Print 200 points on the image For ($ I = 0; I I <200; $ I ++ ){ Imagesetpixel ($ im, rand () % 100, rand () % 30, $ te1 ); } For ($ I = 0; $ I <4; $ I ++ ){ // Dechex () converts decimal to hexadecimal $ Rand. = dechex (rand (1, 15 )); } $ _ Session [check_pic] = $ rand; Imagestring ($ im, 5, rand (3,70), rand (3,15), $ rand, $ textcolor ); Header ("content-type: image/jpeg "); Imagejpeg ($ im );

/*
The purpose of generating verification is to prevent unauthorized user registration. This is a first-class verification program.
*/