"Writing “xfis” is text-message shorthand for “porfis” which is a rather teenage-girly way of saying “por favor.” Other “cute” ways of saying “por favor” include saying “porfi” and “porfa.” Porfa seems to be gaining popularity and isn’t limited to “cute talk” anymore.

"In contemporary usage, the “X” is used for the word “por”... It appears to be related to the use of X in maths, where 2 X 3 is written “dos por tres."
https://www.quora.com/Someone-commented-on-my-tweet-Me-lo-pa...