If you're the kind of person that wonders more about the internals of PHP and how it works "under the covers" you'll find this new post from Nikita Popov a good read. It talks about how PHP handles its order of operations in more complex evaluation statements.
At this point many people seem to think that the order in which an expression is evaluated is determined by operator precedence and associativity. But that's not true. Precedence and associativity only tell you how the expressions are grouped.[...] What does this tell us about the order of evaluation? Nothing. Operator precedence and associativity specify grouping, but they do not specify in which order the groups are executed.
He gives a few examples to illustrate his point including multiple increments of the same variable at one time and how it's the "fault" of the compiled variables that were introduced in PHP 5.1. He shows the opcode version of the same PHP userland code and talks briefly about how to avoid this odd functionality in your application.