Something

if (is_numeric($score) && ($score >= 0) && ($score <= 100))
	{
		if ($score >= 90)
		{
			$grade = 'A';
			return $grade;
		}
		elseif ($score >= 80)
		{
			$grade = 'B';
			return $grade;
		}
		elseif ($score >= 70)
		{
			$grade = 'C';
			return $grade;
		}
		elseif ($score >= 60)
		{
			$grade = 'D';
			return $grade;
		}
		else
		{
			$grade = 'F';
			return $grade;
		}
		
	}
	else
	{
		$ErrorMessage = "<strong>Score</strong> must be between 0 and 100";
		return $ErrorMessage;
	}