Something

if (is_numeric($_REQUEST['score']))
	{
		$ErrorMessage = "<strong>Score</strong> field does NOT contain a number.";
	}

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