The biggest strength of PHP for me has always been that it's free, in contrast to a language like .NET. For example, I used ColdFusion and ActionScript both for years, but only because someone else, namely, my boss at the time, paid for the necessary software to use them!
Another strength of PHP for me has been its relative ease of use and learning curve. For example, for better or worse, I have rarely felt the need to write classes, whereas I have with a language like Java or Apple's dark underbelly, Objective-C.
So, I suppose I see PHP's loosely typed structure as both a strength and a weakness, for more speed and less stress, respectively.
One last weakness of PHP that I should mention, keeping in mind that this might simply reflect my ignorance of available options, is the ease of generating documentation for PHP code. I have used phpDocumentor, for example, but also found annoying problems, like when each of the generated HTML pages starts with an error about not specifying a default timezone. Is this really that hard to eliminate this error?
Here are some relevant discussions I found today:
What are some of the advantages of PHP over other programming languages?
What are the horrors of PHP?
Strengths and Weaknesses of various programming languages
But, however good or bad, by whatever measure, PHP seems to a given developer these days, it does remain a popular choice.