PHP: random characters

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/**
 * Generate and return a random characters string
 *
 * Useful for generating passwords or hashes.
 * 
 * The default string returned is 8 alphanumeric characters string.
 *
 * The type of string returned can be changed with the "type" parameter.
 * Seven types are - by default - available: basic, alpha, alphanum, num, nozero, unique and md5. 
 *
 * @param   string  $type    Type of random string.  basic, alpha, alphanum, num, nozero, unique and md5.
 * @param   integer $length  Length of the string to be generated, Default: 8 characters long.
 * @return  string
 */
function random_str($type = 'alphanum', $length = 8)
{
	switch($type)
	{
		case 'basic'	: return mt_rand();
			break;
                case 'alpha'	:
		case 'alphanum'	:
		case 'num'	:
		case 'nozero'	:
                                $seedings             = array();
            	                $seedings['alpha']    = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
            	                $seedings['alphanum'] = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
            	                $seedings['num']      = '0123456789';
            	                $seedings['nozero']   = '123456789';
                
                                $pool = $seedings[$type];
                
				$str = '';
				for ($i=0; $i < $length; $i++)
				{
					$str .= substr($pool, mt_rand(0, strlen($pool) -1), 1);
				}
				return $str;
			break;
		case 'unique'	:
		case 'md5'	:
				return md5(uniqid(mt_rand()));
			break;
	}
}
/*======================================================+
 Usage Example:
 ---------------
    # Generate 10 chars. alphanumeric string
    echo random_str('alphanum', 10);
    
    # Generate 10 chars. alpha string
    echo random_str('alpha', 10);
    
    # Generate 10 chars. numeric string
    echo random_str('num', 10);
    
    # Generate 10 chars. nozero-numeric string
    echo random_str('nozero', 10);
    
    # Generate basic string
    echo random_str('basic');
    
    # Generate unique string
    echo random_str('unique');
    
    # Generate md5 string
    echo random_str('md5');
    
+======================================================*/