A function to generate random string.

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function random_string($n)
{
    $numbers=range(0,9);        //array of digits
    $letters=range("A","Z");    //array of uppercase characters
    $characters=array_merge($numbers,$letters);
    $characters=array_flip($characters);    //switching key-value of array elements

    $string='';

    for($i=0;$i<$n;$i++)
    {
        $string .= array_rand($characters);     //returns random key from array and adds to $string
    }

    return $string;
}