Loop and Return in PHP

Let's say we have a function that returns the sum of numbers from 1 to 5:

<?php
	function func() {
		$sum = 0;
		
		for ($i = 1; $i <= 5; $i++) {
			$sum += $i;
		}
		
		return $sum;
	}
	
	$res = func();
	echo $res; // outputs 15
?>

Now let's place the return inside the loop, like this:

<?php
	function func() {
		$sum = 0;
		
		for ($i = 1; $i <= 5; $i++) {
			$sum += $i;
			return $sum;
		}
	}
	
	$res = func();
	echo $res;
?>

In this case, the loop will run for only one iteration and an automatic exit from the function (and consequently from the loop) will occur. And in one iteration of the loop, the variable $sum will contain only the number 1, not the entire required sum.

What will be output to the screen as a result of executing the following code:

<?php
	function func($num) {
		$sum = 0;
		
		for ($i = 1; $i <= $num; $i++) {
			$sum += $i;
			return $sum;
		}
	}
	
	echo func(5);
?>

Explain why.

What did the author of this code intend to do? Correct the author's mistake.