The main problem is the popularity of PHP. There are some great PHP developers out there, but because of the popularity many of them are not good. And you learn how to code from the people around you.
The other problem is that both Javascript and PHP encourage implicit casts, which are great but lead to subtle issues you probably won't understand unless you learn a stricter language.
And seemingly the good ones tend to know more languages than just PHP. PHP is just another tool in their toolbox unlike the masses of PHP monkeys out there that no nothing but PHP.
The other problem is that both Javascript and PHP encourage implicit casts, which are great but lead to subtle issues you probably won't understand unless you learn a stricter language.