PHP loop to read from SQLite and write to arrays/JSON

There are more than one error in your code:

$arDaniel = array();
$arAndi = array();
$arDave = array();
$arSimon = array();

for ($j=0; $j < 4; $j++) { 
    $name;                                              # <---
    $arr;                                               # <---

This sort of ‘declaration’ in php is unnecessary, you can remove $name and $arr (it's not an error, btw);

    if (j == 0) {                                       # <---

In the for loop you use $j, you can't use j here: replace it with $j (in php variables are ever prepended by $);

        $name = 'Daniel';
        $arr = $arDaniel;                               # <---

By this assignment, you want change the original array, but with this assignment by value, you in fact create a copy of $arDaniel, and new elements are added only to $arr, not to $arDaniel; to do this, you have to do an assignment by reference through & keyword: replace this (and following similar) line with $arr = &$arDaniel;

    }
        elseif (j == 1) {                               # <--- see above
        $name = 'Andi';
        $arr = $arAndi;                                 # <--- see above
    }
        elseif (j == 2) {                               # <--- see above
        $name = 'Dave';
        $arr = $arDave;                                 # <--- see above
    }
        elseif (j == 3) {                               # <--- see above
        $name = 'Simon';
        $arr = $arSimon;                                # <--- see above
    }
    $query = "SELECT Datum, ID, RR, RL, KB, BD, SD, KH, Reihenfolge FROM $name ORDER BY date(Datum)";
    $i = 1;                                             # <--- 

The query is formally correct, but I don't know your table structure. $i = 1; is unnecessary (see below);

    foreach($res = $db->query($query) as $value) {
        $curr = array();
        array_push($curr["Datum"] = $value[0]);         # <---
        array_push($curr["ID"] = $value[1]);            # <---
        array_push($curr["RR"] = $value[2]);            # <---
        array_push($curr["RL"] = $value[3]);            # <---
        array_push($curr["KB"] = $value[4]);            # <---
        array_push($curr["BD"] = $value[5]);            # <---
        array_push($curr["SD"] = $value[6]);            # <---
        array_push($curr["KH"] = $value[7]);            # <---
        array_push($curr["Reihenfolge"] = $value[8]);   # <---
        array_push($arr[$i] = $curr);                   # <---
        $i++;                                           # <---
    }
}

The array_push syntax is not correct (see manual page): the correct syntax is array_push( $existentArray, $newElement ). In addition, you can't use array_push to add an associative value, you can use it only for numeric key. Change $curr assignments simply in $curr['Datum'] = $value[0]; etc...

To append $curr to $arr, you have to change the line in array_push( $arr, $curr ) or (better, as php recommends if only one element is appended) $arr[] = $curr;. After these changes, the $i++; line can be deleted.

$json = array(
    "Daniel" => $arDaniel,
    "Andi" => $arAndi,
    "Dave" => $arDave,
    "Simon" => $arSimon
);

echo json_encode($json);

$db = NULL;

Now your script is clean (I hope).

Your script remain a bit redundant. You can simplify it in this way:

$array = array( 'Daniel'=>array(), 'Andi'=>array(), 'Dave'=>array(), 'Simon'=>array() );
foreach( $array as $key => &$val )
{
    $query = "SELECT Datum, ID, RR, RL, KB, BD, SD, KH, Reihenfolge FROM $key ORDER BY date(Datum)";

    $result = $db->query( $query );
    $val    = $result->fetchAll( PDO::FETCH_ASSOC );
}

$json = json_encode( $array );
echo $json;

By this way, you init an array with names as key, then, in a foreach loop by reference (&$val) you perform a sql query searching in table $key and you fetch all results directly in array (in this example the values are stored as associative arrays, like in your example, but you can use PDO::FETCH_OBJ to store data as object). At the end, you encode the array end echo it.

Edit:

I see a previous answer nearly identical to mine... BTW, I leave mine because I have spent a lot of time to explain various errors...