PHP's loose typing can mask undefined array key access. This leads to warnings (not always errors) and unexpected results. The example shows how to safely access array elements using `isset() ...

PHP Notice: Undefined index. This bug shows how accessing a non-existent array key in PHP leads to a notice (not an error), potentially resulting in unexpected program behavior and subtle bugs.

Is there a PHP function that can compare all 4 arrays, sum the values of duplicate keys and returns an array with all the sums?<BR><BR>Is that well enough explained?