A function inside an if structure

Question

can I put a function in PHP inside a if structure? like this:

Answer 1

I wanted to add an answer here, because there's a caveat that isn't addressed in the other answers.

Based on my testing in 5.3, it appears that functions that are defined inside if structures are defined at runtime, rather than at compile time like other functions.

For example, outside a conditional block, this:

foo();
function foo(){
   echo 'foo!';
}

works fine because the function is defined at compile time, and executed at runtime. But calling the function before defining it while inside a condition block:

if(1){
   foo(); // Call to undefined function!
   function foo(){
      echo 'foo!';
   }
}

Will produce a call to undefined function error, at least as of version 5.3.

But defining the function in an if block before calling it is valid:

if(1){       
   function foo(){
      echo 'foo!';
   }
   foo(); // No errors, since function is defined!
}

So there is a major gotcha to be aware of when defining functions inside a conditional: the function must be defined in the code before calling the function, since conditional functions don't seem to get defined at compile time.

The OP also asked about performance. If the conditional function is defined at runtime instead of compile time like most other functions, then this function will not benefit from performance boosters like OpCode caching, which depending on the circumstances, could slow down your application. For example, if your master php file looked like:

if($IsLoggedIn){
   // include files/functions and bootstrap here...
}

Then the entire application might not benefit from OpCode caching at all.