First find thekey.php as per the riddle title.

Hover your mouse over the blood footprints from left to right and you will see words appear. In each set of words, there are numbers spelled out.
There are seven numbers in total. Six sets of words, but one of them contains two numbers.

Start from the beginning of the sentence in thekey.php and walk steps according to the numbers you found, in the order they appear when hovering from left to right. Never go back to the start of the key, just keep going. Do not count spaces.

You also found an Old Letter. It is not important for this riddle, but the year written in the lower right corner may be good to keep in mind.