I left my phone charging for 3 weeks.is that safe?

[raanac]

I accidentally left my phone charging for 3 weeks.will it cause a fire

 

[Titillating]

Probably not. It's not a good idea to do that, but most modern batteries are designed to stop/limit power draw once they're fully charged. Of course, there are other factors that might cause issues (electrical outages, surges, etc) so you should avoid this when possible.