PHP push array using array_push() to same array

Hi,

I have this bit of code:

$fields = array(
    //set for all selections
    'METHOD' => $METHOD,
    'VERSION' => $VERSION,
    'USER' => $USER,
    'PWD' => $PWD,
    'SIGNATURE' => $SIGNATURE,
    'BUTTONCODE' => $BUTTONCODE,
    'BUTTONSUBTYPE' => $BUTTONSUBTYPE,
    'L_BUTTONVAR1' => $L_BUTTONVAR1,
    'OPTION0NAME' => $OPTION0NAME
);


$aFieldsAppend = array (
    //set in 3, 6, 12,
    'L_BUTTONVAR2' => $L_BUTTONVAR2,
    'BUTTONTYPE' => $BUTTONTYPE,
    'L_OPTION0SELECT0' => $L_OPTION0SELECT0,
    'OPTION0TYPE' => $OPTION0TYPE
);
array_push($fields, $aFieldsAppend);

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I'm trying to use the array_push() and push another array with key=>value to the same array but when I use it the second array $aFeildsAppend it's pushed to $fields[0] rather than just $fields.

I tried this code too but it doesn't work:

array_push($fields, ('L_BUTTONVAR2' => $L_BUTTONVAR2,
    'BUTTONTYPE' => $BUTTONTYPE,
    'L_OPTION0SELECT0' => $L_OPTION0SELECT0,
    'OPTION0TYPE' => $OPTION0TYPE));

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How can I push the second array to the first one properly so it's all on the same array?

Thanks,
Victor