> Instead of fearing the overhead in PHP for function calls,
Already in 2015 in almost all applications this is a premature optimization which is rightly known as the root of all evil. Please. Your app will talk to the network, likely run a database query which is like thousands or millions of times slower than a function call. Further, even if there is a measurable difference the cost of hardware which makes that difference go away is very likely to be smaller and much smaller at that than the cost of the engineering hours wasted during maintenance when wrestling with code which was written with a "function calls are expensive" mindset.
This doesn't mean you don't need to worry about performance and scalability but even that is going to be much easier if you have a well structured code.