array_map inline anonymous function

Solution 1:

I hope this will help:

$user_meta = array_map(function ($a) { return $a[0]; }, $user_meta);

Solution 2:

There's nothing wrong with the array_map line, but everything before it is wrong. That is the output of a print_r not PHP code. Compare how you define the array in the two links you posted.

Solution 3:

That's not an answer to your question, but since you want to return the first key of each sub-array, you can just use array_column.

$user_meta = array_column($user_meta, 0);

Solution 4:

Slightly shorter could be

$user_meta = array_map(fn ($a) => $a[0], $user_meta);

But I would prefer the array_column approach for such an array_map