Евгений
$user_list = [];
while ($row = mysqli_fetch_assoc($res)) {
$k++;
$a = ;
$b = ;
$c = ;
$d = ;
$user_list[] = [ 'a' => $row['userid'],
'b' => $row['description'],
'c' => $row['username'],
'd' => $row['category'],
'username' => explode(' ', trim($row['username'])
];
}
$user_list = [];
while ($row = mysqli_fetch_assoc($res)) {
$a = $row['userid'];
$b = $row['description'];
$c = $row['username'];
$d = $row['category'];
$user_list[] = [ 'a' => $row['userid'],
'username' => explode(' ', trim($row['username']) )];
}