The general rule is to declare every variable as late as possible.
If the value of a variable doesn't need to be kept across iterations of a loop then declare it inside the loop, or as the loop control variable for a for
loop.
If it needs to remain static across the loop iterations (like your $flag
) then declare it immediately before the loop.
Yes, there is a minimal speed cost to be paid if you discard and reallocate a variable every time a block is executed, but programming and maintenance costs are by far the most important efficiency and should always be put first.
You shouldn't be optimising your code before it has been made to work and found to be running too slowly; and even then, moving declarations to the top of the file is a long way down the list of compromises that are likely to make a useful difference.