PHP append array keys of multidimensional array to a single dimensional array

I am working with large product data API. I am getting a large number of product data in the form of a Multi dimension array.

In my code, I have 130 Categories. Every category has some number of Products listed under it. The product listing I am getting is as below (I copied some portion of my array):

    Array
(
    [0] => Array
        (
            [0] => Array
                (
                    [product_id] => 10295
                    [sku] => BDK Motor
                    [name] => Motor 2005 VW Golf V 5 1K 2,0 SDI Diesel BDK 75 PS 55 KW
                )

            [1] => Array
                (
                    [product_id] => 10502
                    [sku] => BDK Motor-2
                    [name] => Motor 2005 VW Golf V 2,0 SDI BDK 55 KW 75 PS 
                )

            [2] => Array
                (
                    [product_id] => 15693
                    [sku] => CAY Motor
                    [name] => Motor 2009 VW Audi Polo Touran Roomster Golf Leon 1,6 TDI CAY CAYA DE15693
                )

            [3] => Array
                (
                    [product_id] => 17662
                    [sku] => AGR Motor 
                    [name] => Motor 1999 VW Audi Skoda Seat Golf A3 8L Octavia 1,9 TDI Diesel AGR
                )

            [4] => Array
                (
                    [product_id] => 18980
                    [sku] => ZJ-VEM Motor 
                    [name] => Motor 2009 Mazda 2 II DE 1,3 Benzin 55 KW 75 PS ZJ-VEM
                )
    [1] => Array
        (
            [0] => Array
                (
                    [product_id] => 18245
                    [sku] => 08649262 Motorhalter-4
                    [name] => Motorhalter Ford Focus II DA 2,5 ST 166 KW 225 PS HYDA 08649262 SCHROTT
                )

            [1] => Array
                (
                    [product_id] => 24041
                    [sku] => 8D0199335N Halter 
                    [name] => Halter Anschlag Audi 80 A4 Seat Ibiza VW Caddy Passat Golf 1,9 TDI 1Z 8D0199335N
                )

            [2] => Array
                (
                    [product_id] => 24472
                    [sku] => 9639593380 Halter-1
                    [name] => Halter Peugeot Citroen Berlingo C5 Xantia 206 307 2,0 HDI RHY DW10TD 9639593380
                )

            [3] => Array
                (
                    [product_id] => 24606
                    [sku] => 2247711 Motorhalter-3
                    [name] => Motorhalter Halter Rover MG ZT ZT-T 75 RJ 2,0 CDTi 131 PS 204D2 2247711
                )

            [4] => Array
                (
                    [product_id] => 24613
                    [sku] => 204D2 Halter-5 
                    [name] => Halter Halterung Träger Rover MG ZT ZT-T 75 2,0 CDTi 131 PS 204D2
                )

And this array lasts up to 75 array keys with each key having average 80-90 products.

I want to get output like if key[0] have 100 items and key[1] have 90 items so in resultant array after 1st 100 items next items of key[1] appends as [101]; [102];....and so on.

To clear my problem, I need output like below:

Array(
[0]=>
[1]=>
[2]=>
.
.
.
.
[109]=>
.
.
.
[nth key]=>)