How do I add Key=>Value pairs, based on a variable number of Keys, to the 2nd Dimension of an existing 2D Array in PHP?

I need to add additional Key=>Value pairs, based on a variable number of Keys, to and existing 2D Array in PHP.

I have spent days on this and very much need a solution.

As you can see there are 19557 Arrays in the second dimension.

I need it add Key=>Value pairs begging at index '5' up to a variable number.

Here is the structure of the array.

array(19557) { 
	[0]=> array(5) { 
		[0]=> string(6) "160478" 
		[1]=> string(21) "2019-01-01-2019-01-15" 
		[2]=> string(2) "90" 
		[3]=> string(5) "10217" 
		[4]=> string(4) "ESPC" 
	} 
	
	[1]=> array(5) { 
		[0]=> string(6) "160478" 
		[1]=> string(21) "2019-01-01-2019-01-15" 
		[2]=> string(2) "90" 
		[3]=> string(5) "10217" 
		[4]=> string(3) "PIP" 
	} 
	
	[2]=> array(5) { 
		[0]=> string(6) "160479" 
		[1]=> string(21) "2019-01-01-2019-01-15" 
		[2]=> string(2) "90" 
		[3]=> string(5) "10360" 
		[4]=> string(5) "FDNSC" 
	}
	...
}

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Can anyone help?

It would be greatly appreciated.