What is PHP and how serious is the computer language flaw causing website shutdowns?

An estimated 70% of websites use the open-source language. What is the identified vulnerability in it?

Flaw in PHP computer language leads to shutdown of websites around world

Precautionary takedowns come as experts say vulnerability in PHP, which is used by 70% of sites, could be exploited ...

Tracing the History of Computer Languages: From "What is PHP?" and "What is a Language?"

If you understand this, you won't be afraid even if you can't use it!70 Years of Computer Language History from FORTRAN to ...