PHP image Verification Code Program (letters and numbers)

<? PHP
// Filename: authimg. php
// Description:
// Creater: Alvar
// Createtime: 2006-5-4
// Lastmodtime:

Session_start ();
?>
<? PHP
// Generate a verification code Image
Header ("Content-Type: image/PNG ");
Srand (double) microtime () * 1000000); // play the next seed that generates random numbers to facilitate the use of the following Random Number Generation

// Session_start (); // Save the random number to the session.
$ _ Session ['authnum'] = "";
$ Im = imagecreate (70, 20) or die ("cant's initialize new GD image stream! "); // Specifies the image background size.
$ Black = imagecolorallocate ($ im, 0, 0); // you can specify three colors.
$ White = imagecolorallocate ($ im, 255,255,255 );
$ Gray = imagecolorallocate ($ im, 200,200,200 );

Imagefill ($ im, $ gray); // use the area filling method, set)

// Generate a verification code mixed with numbers and letters
$ Ychar = ", A, B, C, D, E, F, G, H, I, J, K, L, M, n, O, P, Q, R, S, T, U, V, W, X, Y, Z ";
$ List = explode (",", $ ychar );
For ($ I = 0; $ I <4; $ I ++ ){
$ Randnum = rand (0, 35 );
$ Authnum. = $ list [$ randnum];
}

// While ($ authnum = rand () % 100000) <10000); // generate a random number of five students
// Print the five-digit integer verification code into the image
$ _ Session ['authnum'] = $ authnum;

Imagestring ($ im, 5, 10, 3, $ authnum, $ black );
// Use the col color to draw the string s to the X and Y coordinates of the image (0 and 0 in the upper left corner of the image ).
// If the font is 1, 2, 3, 4, or 5, use the built-in font.

For ($ I = 0; $ I <200; $ I ++) // Add interference pixels
{
$ Randcolor = imagecolorallocate ($ im, Rand (0,255), Rand (0,255), Rand (0,255 ));
Imagesetpixel ($ im, Rand () % 70, Rand () % 30, $ randcolor );
}
Imagepng ($ IM );
Imagedestroy ($ IM );

?>