How to solve: Notice: Undefined variable

Happens when you try to use a variable that wasn't previously defined.

A typical example would be

foreach ($items as $item) {
    // do something with item
    $counter++;
}

If you didn't define $counter before, the code above will trigger the notice.

The correct way would be to set the variable before using it, even if it's just an empty string like

$counter = 0;
foreach ($items as $item) {
    // do something with item
    $counter++;
}