If you take one of these good developers, nothing much, other than that they might need to write some things that they wouldn't have to think about in other languages.
If you take a developer who isn't as good, or isn't as experienced, and they create a ball of mud, it's easier to get it to a maintainable point by a good dev if it's written in a language other than PHP.
Some things about PHP that make it difficult to maintain over time are things like lack of consistency in abstraction, lack of first-class objects, extreme behavioral dependency on php.ini, incomplete reflection capabilities, and so on. These types of things are much bigger of a deal when trying to refactor a messy codebase as opposed to developing on a pretty clean one. They're getting better as time goes on, but the other popular web-dev languages are miles ahead, and it's extremely likely that a good dev capable of writing PHP code is familiar with one of them.
If you take a developer who isn't as good, or isn't as experienced, and they create a ball of mud, it's easier to get it to a maintainable point by a good dev if it's written in a language other than PHP.
Some things about PHP that make it difficult to maintain over time are things like lack of consistency in abstraction, lack of first-class objects, extreme behavioral dependency on php.ini, incomplete reflection capabilities, and so on. These types of things are much bigger of a deal when trying to refactor a messy codebase as opposed to developing on a pretty clean one. They're getting better as time goes on, but the other popular web-dev languages are miles ahead, and it's extremely likely that a good dev capable of writing PHP code is familiar with one of them.