<?php
$string = 'United States, Canada, France, Germany, United Kingdom';
$allowed = array(
    'United States' => 'US',
    'Canada' => 'CA',
    'United Kingdom' => 'UK'
);
    
$array = explode(', ',$string);

foreach($array as $key => $country){
    if(array_key_exists($country,$allowed)){
        $array[$key] = $allowed[$country];
    }
    else{
        unset($array[$key]);
    }
}

$string = implode(', ',$array);
var_dump($string);
?>

string(10) "US, CA, UK"