How to create random string and check if it exists form database , if it does create a new one using php.You can use php time() function to create random string and convert it using md5() function.

function rand_string(){
    $string = substr(md5(time()), 0, 5);
    if(exists_in_db($string)) {
		// check in the db if exist, call again to this function
		 $string  = rand_string();
	}
    return $string ;
}

 
How to call to this function to get value.

  $random = rand_string();

Note:exists_in_db() is a function.You must write this function after getting parameter and it check in the database and return value.

function exists_in_db($val) {
	// get database connection first, then do this
	$sql = "SELECT id FROM table WHERE rand_val = '$val'";
	$result = mysql_query($sql) or die(mysql_error());
	$num_rows = mysql_num_rows($result) or die(mysql_error());
	if($num_rows>0) {
		// exist this value in the db
		return true;
	}
	else {
		// not exist in the db
		return false;
	}
}