array_push for associative arrays
This is your problem:
$newArray[$key] is null cause $newArray is an empty array and has not yet values.
You can replace your code, with
array_push( $newArray, $value );or instead of array_push to use
$newArray[$key] = $value;so you can keep the index of your $key.
I use array_merge prebuilt function for push in array as associative.
For example:-
$jsonDataArr=array('fname'=>'xyz','lname'=>'abc');$pushArr=array("adm_no" => $adm_no,'date'=>$date);$jsonDataArr = array_merge($jsonDataArr,$pushArr);print_r($jsonDataArr);//Array ( [fname] => xyz [lname] => abc [adm_no] =>1234 [date] =>'2015-04-22')