<?php
foreach ($a as $key => $row)
{
$id[$key] = $row[0];
}
Standard input is empty
Array
(
    [0] => Array
        (
            [0] => idA
            [1] => usernameA
        )
    [1] => Array
        (
            [0] => idB
            [1] => usernameB
        )
    [2] => Array
        (
            [0] => idC
            [1] => usernameC
        )
)