$arr = [

"I4" => 7 , "I3" => 7 ,
"I2" => 7 , "I1" => 8 ,

"B4" => 8 , "B3" => 8 ,
"B2" => 8 , "B1" => 9 ,

"S4" => 10 , "S3" => 10 ,
"S2" => 10.50 , "S1" => 12 ,

"G4" => 15 , "G3" => 15.50 ,
"G2" => 16 , "G1" => 17 ,

"P4" => 27 , "P3" => 28 ,
"P2" => 30 , "P1" => 35 ,

"D4" => 70 , "D3" => 80 ,
"D2" => 120 , "D1" => 180

];

$select1 = "I4";
$select2 = "I1";


$p1 = array_search($select1 ,array_keys($arr));
$p2 = array_search($select2 ,array_keys($arr));

$keys = array_keys($arr);