That and the ease of deployment, which is possibly the strongest factor. Most hosting companies will provide you with PHP on apache even if you don't ask for it.
I'd say that greatest data structure in all languages is PHP array().
It's actually an ordered dictionary with default implicit natural number keys. Efficient enough to represent concepts such as arrays, sets, queues, stacks, trees, graphs without bothering with hashtables, linked lists, reallocations and other underlying data structures tailored for specific purpose.
Nope. I honestly think that that this is one of the main reasons PHP is popular and is able to power even the most complex websites with ease.
Before PHP I did Basic (Atari), 6502 Assember, Logo, ACTION!, Pascal (Kyan, Turbo), C++ (Turbo, Borland) , Java, C#. After PHP I did JavaScript and Python.
First I've met PHP-like arrays in awk and was astonished with its versatility.
I still believe that data structure that allows person to make a queue, stack, tree, even graph easily without knowing any theory and planning anything is one of the greatest things I've encountered in programming languages.
I am very dissatisfied that python does not have standard ordered dictionary and strongly differentiates between tuple, array, set and dict even though they provide just overlapping subsets of odict functionality.
These are I think its two strongest points.