Как преобразовать эту строку в массив?

Так как ваш $response['message'] содержит строку, которую вы хотели бы преобразовать в массив, вы можете использовать взорвать функцию:

$exploded_string_array = explode("\n", $response['message']);
print_r($exploded_string_array); //to dump the array to the screen and u see how it was exploded.

Это позволит вам получить доступ к массиву, используя для каждого:

foreach ($exploded_string_array as $key => $value) {
echo $key ." ". $value;
}

Обновить:

После того, как вы обновили свой вопрос и сделали его более понятным, вот один из способов получить желаемый результат:

<?php
//Updated the code to use array built-in functions instead of manually setting the keys.
$stringToExplode = "\"ADGROUP_PERFORMANCE_REPORT (Jan 11, 2018-Jan 17, 2018)\"\nAd group ID,Ad group,Ad group state,Campaign ID,Campaign,Campaign state\n47069225942,Conversse,enabled,1002620524,Blizzard Entertainment #5a20e1cd04140,enabled\n49685186135,Starcraft II #5a20e20434e4b,enabled,1002620524,Blizzard Entertainment #5a20e1cd04140,enabled\n52639270027,midnight test,enabled,1002620524,Blizzard Entertainment #5a20e1cd04140,enabled\n50529526715,one,enabled,1010615735,December,enabled\n50530330478,Don't Remove ;),enabled,823386372,BS Test Campaign,enabled\n52012184360,Don't Remove ;),enabled,869044770,BruceTesting,enabled\n50530808398,SAMPLE KEN 2 #5a1fb34fb81c8,enabled,999354939,lazada,enabled\n53247341001,ken 4,paused,999354939,lazada,enabled\n53451857190,SAMPLE KEN 3 #5a1fc416984ce,enabled,999354939,lazada,enabled\n54964303332,SAMPLE KEN 1 #5a1fb3321af4c,enabled,999354939,lazada,enabled\n57947888068,ken5,enabled,999354939,lazada,enabled\n52357571231,Beyond Science Free Trial,enabled,1007003986,Gavin Testing,paused\nTotal, --, --, --, --, --\n";

$exploded_string = explode("\n", $stringToExplode);
$inner_exploded_array = [];

foreach ($exploded_string as $key => $value) {
array_push($inner_exploded_array, explode(",", $value));
}

$final_array = [];

for ($j=2;$j<count($inner_exploded_array)-2;$j++) {
$inner_array = array_combine($inner_exploded_array[1], $inner_exploded_array[$j]);
array_push($final_array, $inner_array);
}

echo "<pre>";
print_r($final_array);
echo "</pre>";
?>

Выход:

Array
(
[0] => Array
(
[Ad group ID] => 47069225942
[Ad group] => Conversse
[Ad group state] => enabled
[Campaign ID] => 1002620524
[Campaign] => Blizzard Entertainment #5a20e1cd04140
[Campaign state] => enabled
)

[1] => Array
(
[Ad group ID] => 49685186135
[Ad group] => Starcraft II #5a20e20434e4b
[Ad group state] => enabled
[Campaign ID] => 1002620524
[Campaign] => Blizzard Entertainment #5a20e1cd04140
[Campaign state] => enabled
)

[2] => Array
(
[Ad group ID] => 52639270027
[Ad group] => midnight test
[Ad group state] => enabled
[Campaign ID] => 1002620524
[Campaign] => Blizzard Entertainment #5a20e1cd04140
[Campaign state] => enabled
)

[3] => Array
(
[Ad group ID] => 50529526715
[Ad group] => one
[Ad group state] => enabled
[Campaign ID] => 1010615735
[Campaign] => December
[Campaign state] => enabled
)

[4] => Array
(
[Ad group ID] => 50530330478
[Ad group] => Don't Remove ;)
[Ad group state] => enabled
[Campaign ID] => 823386372
[Campaign] => BS Test Campaign
[Campaign state] => enabled
)

[5] => Array
(
[Ad group ID] => 52012184360
[Ad group] => Don't Remove ;)
[Ad group state] => enabled
[Campaign ID] => 869044770
[Campaign] => BruceTesting
[Campaign state] => enabled
)

[6] => Array
(
[Ad group ID] => 50530808398
[Ad group] => SAMPLE KEN 2 #5a1fb34fb81c8
[Ad group state] => enabled
[Campaign ID] => 999354939
[Campaign] => lazada
[Campaign state] => enabled
)

[7] => Array
(
[Ad group ID] => 53247341001
[Ad group] => ken 4
[Ad group state] => paused
[Campaign ID] => 999354939
[Campaign] => lazada
[Campaign state] => enabled
)

[8] => Array
(
[Ad group ID] => 53451857190
[Ad group] => SAMPLE KEN 3 #5a1fc416984ce
[Ad group state] => enabled
[Campaign ID] => 999354939
[Campaign] => lazada
[Campaign state] => enabled
)

[9] => Array
(
[Ad group ID] => 54964303332
[Ad group] => SAMPLE KEN 1 #5a1fb3321af4c
[Ad group state] => enabled
[Campaign ID] => 999354939
[Campaign] => lazada
[Campaign state] => enabled
)

[10] => Array
(
[Ad group ID] => 57947888068
[Ad group] => ken5
[Ad group state] => enabled
[Campaign ID] => 999354939
[Campaign] => lazada
[Campaign state] => enabled
)

[11] => Array
(
[Ad group ID] => 52357571231
[Ad group] => Beyond Science Free Trial
[Ad group state] => enabled
[Campaign ID] => 1007003986
[Campaign] => Gavin Testing
[Campaign state] => paused
)

)