Set sign text

How do I set a text on a sign when a player taps it, (what function)?

Function?!
If event - use PlayerInteractEvent.

I know that but I'm wanting to set the text when the player interacts with a sign.

Look on this, it will help you
http://docs.pocketmine.net/dc/de2/classpocketmine_1_1tile_1_1_sign.html

You needed function - setText();

I want thinking of that function but I wasn't sure, thank you anyway.