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This is a function, that will return false if the string passed to it is not alphanumeric, and true if it is.

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<?

function alphanumeric($string) {
    $alphachars[0] = "9";
    $alphachars[1] = "a";
    $alphachars[2] = "b";
    $alphachars[3] = "c";
    $alphachars[4] = "d";
    $alphachars[5] = "e";
    $alphachars[6] = "f";
    $alphachars[7] = "g";
    $alphachars[8] = "h";
    $alphachars[9] = "i";
    $alphachars[10] = "j";
    $alphachars[11] = "k";
    $alphachars[12] = "l";
    $alphachars[13] = "m";
    $alphachars[14] = "n";
    $alphachars[15] = "o";
    $alphachars[16] = "p";
    $alphachars[17] = "q";
    $alphachars[18] = "r";
    $alphachars[19] = "s";
    $alphachars[20] = "t";
    $alphachars[21] = "u";
    $alphachars[22] = "v";
    $alphachars[23] = "w";
    $alphachars[24] = "x";
    $alphachars[25] = "y";
    $alphachars[26] = "z";
    $alphachars[27] = "0";
    $alphachars[28] = "1";
    $alphachars[29] = "2";
    $alphachars[30] = "3";
    $alphachars[31] = "4";
    $alphachars[32] = "5";
    $alphachars[33] = "6";
    $alphachars[34] = "7";
    $alphachars[35] = "8";
    for ($i=0;$i<strlen($string);$i++) {
        for ($j=0;$j<36;$j++) {
            if(strtolower(substr($string, $i, 1)) == $alphachars[$j]) {
                $charok == 1;
            }
        }
        if (!$charok) {
            return false;
        }
        $charok = NULL;
    }
    return true;
}

?>