You may get it this way
$value = $countries[$country_code];
According to php documentation:
Array elements can be accessed using the array[key] syntax.
In your code it will looks like: $value = $countries[$country_code];
Also I recommend you to read about arrays in PHP here: http://php.net/manual/en/language.types.array.php your case is explained in 6th example.
just to expand on @B.Mossavari answer, you should check the key is there before extracting the value or PHP will return a undefined index Notice
if (array_key_exists($country_code, $countries)) {
$value = $countries[$country_code];
} else {
$value = ''; // set value to something so your code doesn't fail later
}
this is my preferred way, but you could also check using isset($countries[$country_code]) or !empty($countries[$country_code])
$value = (array_key_exists($country_code, $countries)) ? $countries[$country_code] : ''
$countries[$country_code]??null, this will return null if there isn't one in the array.