$user = array(
    'name' => 'John',
    'age' => 35
);

// echo key($user)." is ".($user[key($user)]).",";

$counter = count($user);
$i = 1;
$res = '';

foreach ($user as $key => $value) {
    $res .= $key. " is ". $value;
    if ($counter != $i) {
        $res = $res .", ";
    }
    $i = $i + 1;
}
echo $res;

$values =[];

foreach ($user as $key => $value) {
    $values[] = "{$key} is {$value}";
}
echo implode(', ', $values);

ou

$values = array_map(fn($key, $value) => "{$key} is {$value}", array_keys($user), $user);
echo implode(', ', $values);