php - I'm receiving Undefined offset error -

why receive undefined offset error? i'm trying add 10,20,20 each element in array. please help. in advance

<?php     $arr = array("a","b","c");     $counter = 0;     $status = array();     foreach($arr $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 

`

in code, $status empty array, when attempt add undefined index see notice (only first time).

to initialize $status array values 0 based on number of elements in $arr:

$status = array_fill(0, count($arr), 0);