Array loop traversal print results Little brother I did not understand when I was learning, to ask you next

Array loop traversal print out results I didn't read it when I was studying, and I asked you for advice.
When I was learning PHP, I saw the array looping through the printout, and there was a sentence that didn't read.
The code is as follows

$arr [0]=1;
$arr [1]=2;
$arr [2]=3;
$arr [3]=4;
Cycle start
for ($i =0; $i <4; $i + +) {
echo $i. $arr [$i]; May I ask you $arr[$i] This sentence is what meaning, why to put the loop $i this variable, put into the array $arr inside go
}

------to solve the idea----------------------
They're all subscript and PHP subscript is starting from 0 is subscript 0 corresponds to the first number 1 corresponds to a second number and so on
------to solve the idea----------------------
This is the loop, when $i=0, $i =1, $i =2, $i = 3 o'clock, respectively output $ARR[0], $arr [1], $arr [2], $arr [3] value.
------to solve the idea----------------------
Looking at the definition of the array, the subscript represents the position of the array value at the index.

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