for循环不会遍历PHP中的整个数组

I use PHP/7.2.0beta3. So I want to create a custom function to reverse an array in PHP. If the array is (1,2,3) the functions turns it to (3,2,1).

I thought I should use the array_pop, grab the last value of the array and pass it to another array.

The problem

Here is the code I wrote. Just copy it and run it as PHP. I dont know why it stops in the middle of the array and does not continue till the end.

$originalarray = range(0, 100, 5);//works
echo '<br> original array <br>';
print_r($originalarray); // 0-100, with 5 step

function customreverse($x){
    echo '<br> original array in function <br>';
    print_r($x); //works, 0-100, with 5 step
    echo '<br> sizeof in function '.sizeof($x).'<br>'; //works, is 21
    for($a=0; $a<sizeof($x); $a++){
        $reversearray[$a] = array_pop($x);
        echo '<br> reversearray in for loop <br>';
        print_r($reversearray);//stops at 50
        echo '<br> a in for loop <br>';
        echo $a;//stops at 10
    }   

    echo '<br> reverse in function <br>';
    print_r($reversearray);////stops at 50
}
customreverse($originalarray);

The same problem occurs even if I replace sizeof with count. Or $a<sizeof($x) with $a<=sizeof($x). Why does it stop and does not traverse the whole array? What am I missing here?

Thanks

sizeof (or count) is evaluated on every iteration of the loop and the array shrinks on each iteration. You need to store the original count in a variable. For Example (I removed a few lines to focus on the issue):

<?php
$originalarray = range(0, 100, 5);//works

function customreverse($x){
  $origSize=sizeof($x);
  for($a=0; $a<$origSize; $a++){
    $reversearray[$a] = array_pop($x);
  }   
  return($reversearray);//stops at 50 (Now it doesn't)
}
print_r(customreverse($originalarray));

jh1711 properly explained that your loop ends early because the middle statement in for(statement1; statement2; statement3) gets executed each iteration, and because you're popping the original array within the loop, sizeOf() returns a smaller number each time.

You could compact your code a bit, by building your reverse array like so:

while(!empty($original)) $reverse[] = array_pop($original);

If you want to preserve key=>value bindings (meaning reverse keys as well, so that the same keys will bind to the same values), you could do:

while(!empty($original)):
  $val = end($original); // set pointer at end of array
  $reverse[key($original)] = $val;
endwhile;

If you want to modify the array in-place (not create a 2nd array), you could do:

for($i=0, $j=sizeof($original); $i < $j; $i++){
    array_splice($original,$i,0,array_pop($original));
} // pop last element and insert it earlier in the array