Unnecessary Array Length Calculation in a Loop in PHP

Let there be a given array with numbers:

php<?php
  $arr = [1, 2, 3, 4, 5];
?>

Let's make it so that each element of the array is squared. That is, we should have the same array $arr, but with squared numbers.

In this case, the foreach loop is not suitable, and we need a regular for loop for the array, like this:

php<?php
  $arr = [1, 2, 3, 4, 5];
  
  for ($i = 0; $i < count($arr); $i++) {
    $arr[$i] = $arr[$i] * $arr[$i]; // change each array element
  }
  
  var_dump($arr);
?>

What's wrong with this code? The problem is that count($arr) will be pointlessly calculated on every loop iteration! Why, because the array length doesn't change during the loop, right?

To optimize, let's move the calculation of the array length into a separate variable:

php<?php
  $arr = [1, 2, 3, 4, 5];
  $len = count($arr); // optimize
  
  for ($i = 1; $i < $len; $i++) {
    $arr[$i] = $arr[$i] * $arr[$i];
  }
  
  var_dump($arr);
?>

php<?php
  $arr = [1, 2, 2, 4, 5];
  
  for ($i = 1; $i < count($arr); $i++) {
    if ($arr[$i - 1] === $arr[$i]) {
      echo $arr[$i] . '<br>';
    }
  }
?>