php ajax返回整个页面

<?php
header('Content-Type: application/json');

if ($_SERVER['REQUEST_METHOD'] === 'POST') {
    // 处理POST请求
    $data = array(
        'status' => 'success',
        'message' => 'Data received successfully'
    );
    echo json_encode($data);
} else {
    // 处理GET请求
    $data = array(
        'status' => 'success',
        'message' => 'Data fetched successfully'
    );
    echo json_encode($data);
}
?>

function sendAjaxRequest() {
    var xhr = new XMLHttpRequest();
    xhr.open('POST', 'ajax_handler.php', true);
    xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
    xhr.onreadystatechange = function() {
        if (xhr.readyState === 4 && xhr.status === 200) {
            var response = JSON.parse(xhr.responseText);
            console.log(response.message);
        }
    };
    xhr.send('key=value');
}