I hope this will help:
$user_meta = array_map(function ($a) { return $a[0]; }, $user_meta);
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.
array_map
print_r
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.
array_column
$user_meta = array_column($user_meta, 0);