php - renaming array from value

i have a string, say its

ID=14,[email protected]@ID=15,[email protected]@
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i exploded it with

$pieces = explode("@@", $contents);
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so now i have a set of array

array(3) 
{ 
[0]=> string(69) "ID=14,123" 
[1]=> string(9) "ID=15,789" 
[2]=> string(0) "" 
}
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then i would like to explode again the string within the array and used this

foreach ($pieces as $key => $value){
     $pieces[$key] = strpos( $value, "," ) ?
                explode( ",", $value ) :
                $value; 

     }
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now i have an even more nested one

array(3) 
{ 
[0]=> array(2) 
   { 
   [0]=> string(5) "ID=14" 
   [1]=> string(3) "123" 
   } 
[1]=> array(2) 
   { 
   [0]=> string(5) "ID=15" 
   [1]=> string(3) "789" 
   } 
[2]=> string(0) 
   "" 
}
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but what i wanted was so that the word "ID" can be replaced into the key of the array so it becomes

array 
{ 
[ID=14]=> "123" 
[ID=15]=> "789" 
}
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how can that be done? i'm very unfamiliar with array but would love to learn.