错误：SQLSTATE [HY000] [2002]没有这样的文件或目录（PDO）

Question

大家好，我遇到了问题。我有2个php文件

核心php（发生错误的地方）：

<?php
include('getSecondDropDown.php');

$arrayForSecondDropdown[] = functionName();
foreach($arrayForSecondDropdown as $v){
       echo "<option value='" . $v['Country_ID'] ."'>" . $v['Country_Name'] ."</option>";
}
?>

getSecondDropDown.php：

<?php
    include('config.php');
    function functionName(){
    $Region_ID =$_POST['Region_ID'];
    $option="";
              try {
    $conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    $stmt = $conn->prepare("SELECT Country_ID, Country_Name FROM Countries WHERE Region_ID ='".$Region_ID."'"); 
    $stmt->execute();

    $result = $stmt->setFetchMode(PDO::FETCH_ASSOC); 

    foreach($stmt as $v) { 

    echo "<option value='" . $v['Country_ID'] ."'>" . $v['Country_Name'] ."</option>";

}
              }
              catch(PDOException $e) {
    echo "Error: " . $e->getMessage();
}
    }
        echo $option;

  ?>

完整错误代码（在Web控制台中显示 - 来源）：

Country: <select name="Country_ID" id="Country_ID" form="ServiceForm">
Error: SQLSTATE[HY000] [2002] No such file or directory<option value=''></option></select></br>