I think the low salaries are more a reflection of the fact that there are a lot of inexperienced developers writing PHP and that there is a lot of unimportant s

I think the low salaries are more a reflection of the fact that there are a lot of inexperienced developers writing PHP and that there is a lot of unimportant software written in PHP than it is that a given developer would make less by using PHP. A highly talented developer working on business critical software will make a lot more than an untalented developer working on low value software regardless of whether each piece of software is in PHP or Java; it's just that the first scenario is more common in Java than in PHP and the second scenario is more common in PHP than in Java.