<?php
$fruits = [
           'apple'=>'100',
           'orange'=>'150',
           'melon'=>'300',
           'banana'=>'200',
           'pineapple'=>'500',
           'grape'=> 300,
           'peach' => 150
          ];
 
$fruits_key = array_keys($fruits, '300');
 
print_r($fruits_key);

?>