I'm receiving Undefined offset error - php

Question

Why do I receive Undefined offset error? I'm trying to add 10,20,20 for each element in array. Please help. Thanks in advance
<?php
$arr = array("a","b","c");
$counter = 0;
$status = array();
foreach($arr as $a){
$status[$counter] += 10;
$status[$counter] += 20;
$status[$counter] += 20;
echo $status[$counter]."<br>";
$counter ++;
}
?>
Error:
Notice: Undefined offset: 0 in C:\xampp\htdocs\test\index.php on line 6
300
Notice: Undefined offset: 1 in C:\xampp\htdocs\test\index.php on line 6
300
Notice: Undefined offset: 2 in C:\xampp\htdocs\test\index.php on line 6
300
`

Answer 1

you are trying to add 10 in a undefined array element in this line:
$status[$counter] += 10;
try like this:
$arr = array("a","b","c");
$counter = 0;
$status = array();
foreach($arr as $a){
$status[$counter] = 10;//assign first
$status[$counter] += 20; //concate with assigned element
$status[$counter] += 20;
echo $status[$counter]."<br>";
$counter ++;
}
it should not provide any notices.

Answer 2

In your code, $status is an empty array, so when you attempt to add something to an undefined index you will see that notice (only for the first time).
To initialize $status as an array with values 0 based on the number of elements in $arr:
$status = array_fill(0, count($arr), 0);

Answer 3

You're using an 'add and assign' operator.
If you just want to assign a value then
$status[$counter] = 10;
Will work just fine.
However, you're asking PHP to add something to an existing element in your array, but there's no element there yet since you haven't initialised it. Just initialise your array before you start your loop.