mysqli fetch_all() not a valid function?

The typical way to construct an associative array is in a while loop:

$results_array = array();
$result = $mysqli->query($query);
while ($row = $result->fetch_assoc()) {
  $results_array[] = $row;
}

This function is available since PHP 5.3.0. Possibly your version is older. Use fetch_assoc() instead.

while ($row = $result->fetch_assoc()) {
    // do what you need.
}

mysqli::fetch_all() needs mysqlnd driver to be installed before you can use it.