array within indexed numbers - php

Question

I tried to test a new thing and just saw an unusual output I am unable to get what exactly is happening within the loop can anyone explain me what happens if we take an array and assign that array to a variable we run a for loop and we provide $array and index of $array with the inititlizer for forloop something like this $array[$array[$i]] so I mean I am confused to totally explain but can you review the code and let me know what exactly is happening
$array = array(1, 2, 3, 5, 8, 13, 21, 34, 55);
$sum = 0;
for($i = 0; $i < 5; $i++) {
$sum += $array[$array[$i]];
}
echo $sum ;
The output 78 as it added the value but if i remove $sum+= and write down like this
echo $array[$array[$i]] . "<br />";
so now what here is I will be getting output like this
2
3
5
13
55
I am unable to get what exactly hapend within this loop

Answer 1

You are using the value of the array to access the key.
$array = array(1, 2, 3, 5, 8, 13, 21, 34, 55);
$sum = 0;
for($i = 0; $i < 5; $i++) {
$sum += $array[$array[$i]] . "<br />";
}
echo $sum ;
Loop 1:
$i = 0;
$sum += $array[$array[0]];
$sum += $array[1];
$sum += 2;
Loop 2:
$i = 1;
$sum += $array[$array[1]];
$sum += $array[2];
$sum += 3;
Loop 3:
$i = 2;
$sum += $array[$array[2]];
$sum += $array[3];
$sum += 5;
Loop 4:
$i = 3;
$sum += $array[$array[3]];
$sum += $array[5];
$sum += 13;
Loop 5:
$i = 4;
$sum += $array[$array[4]];
$sum += $array[8];
$sum += 55;
So...
$sum = 2 + 3 + 5 + 13 + 55 = 80

Answer 2

For $i = 0
what is $array[$i] -> $array[0] -> 1.
what is $array[$array[$i]] -> $array[1] -> 2.
And so on.

Answer 3

First iteration ($i = 0)
$array[$i] = $array[0] = 1
and
$array[1] = 2
Second iteration ($i = 1)
$array[$i] = $array[1] = 2
and
$array[2] = 3
Third iteration ($i = 2)
$array[$i] = $array[2] = 3
and
$array[3] = 5
Fourth iteration ($i = 3)
$array[$i] = $array[3] = 5
and
$array[5] = 13
Fifth iteration ($i = 4)
$array[$i] = $array[4] = 8
and
$array[8] = 55

Answer 4

When $i = 0 $array[$array[$i]] becomes $array[$array[0]] which is $array[1] since $array[0] = 1. So $array[1] evaluates to 2.
Keep thinking the same way.
For example, for $i = 4 $array[$array[$i]] becomes $array[$array[4]] which is $array[8] since $array[4] = 8. So $array[8] evaluates to 55.
Think step by step.