No it isn’t - as the article states, many people write insecure programs using PHP but PHP itself is as secure as most other programming languages.
There is a huge difference between insecurity being built into the language and people simply writing insecure code using a secure language.
One of the biggest problems is that almost all books and sites teaching PHP do not include the code to make their example scripts secure in order to save space and to make the code being discussed easier to see. The people learning mostly assume that the code they have been taught is sufficient and so they leave out the other 80% of the code they should be writing that handles the security aspects.
There are books available on PHP security that specifically teach you how to write the code in a secure manner and if that approach is followed then PHP is as secure as any programming language can be.