根据你的代码

<$php
     $val = $myService->getValue(); // Makes an API and database call
     echo '<span id="value">'.$val.'</span>';
$>

现在你可以使用DOM，使用innerHTML (span id)来获取值，在这种情况下你不需要调用任何服务器，或者Ajax或其他东西。

您的页面将使用PHP打印它，而JavaScript将使用DOM获取值。

myPlugin.start($val); // Tried this, didn't work

它不起作用，因为就JavaScript而言，$val是未定义的，即PHP代码没有为$val输出任何东西。试着在浏览器中查看源代码，下面是你将看到的:

myPlugin.start(); // I tried this, and it didn't work

And

<?php myPlugin.start($val); ?> // This didn't work either

这并不管用，因为PHP将尝试将myPlugin视为常量，当失败时，它将尝试将其视为字符串'myPlugin'，它将尝试与PHP函数start()的输出连接，由于这是未定义的，它将产生一个致命的错误。

And

 myPlugin.start(<?=$val?> // This works sometimes, but sometimes it fails

虽然这是最有可能工作的，因为PHP代码生成了带有预期参数的有效JavaScript，但如果失败，很可能是因为myPlugin还没有准备好。检查你的执行顺序。

您还应该注意到，PHP代码输出是不安全的，应该使用json_encode()进行过滤。

EDIT

因为我没有注意到myPlugin.start(<?=$val?>: \

正如@Second Rikudo指出的那样，要正确工作，$val需要包含右括号，例如:$val="42);"

这意味着PHP现在将生成myPlugin.start(42);并在JavaScript代码执行时按预期工作。

<?php

    $val = $myService->getValue(); // Makes an API and database call

    echo "
        <script>
            myPlugin.start({$val});
        </script> ";

?>

经过大量的研究，我发现最简单的方法是简单地传递各种变量。

在服务器脚本中，你有两个变量，你试图将它们发送到客户端脚本:

$php_var1 ="Hello world";
$php_var2 ="Helloow";
echo '<script>';
echo 'var js_variable1= ' . json_encode($php_var1) . ';';
echo 'var js_variable2= ' . json_encode($php_var2) . ';';
echo '</script>';

在页面上调用的任何JavaScript代码中，只需调用这些变量。

实际上有几种方法可以做到这一点。有些比其他的需要更多的开销，有些被认为比其他的更好。

排名不分先后:

使用AJAX从服务器获取所需的数据。 将数据回显到页面的某处，并使用JavaScript从DOM获取信息。 将数据直接回显到JavaScript。

在这篇文章中，我们将研究上述每种方法，并了解每种方法的优缺点，以及如何实现它们。

1. 使用AJAX从服务器获取所需的数据

这种方法被认为是最好的，因为您的服务器端和客户端脚本是完全独立的。

Pros

Better separation between layers - If tomorrow you stop using PHP, and want to move to a servlet, a REST API, or some other service, you don't have to change much of the JavaScript code. More readable - JavaScript is JavaScript, PHP is PHP. Without mixing the two, you get more readable code on both languages. Allows for asynchronous data transfer - Getting the information from PHP might be time/resources expensive. Sometimes you just don't want to wait for the information, load the page, and have the information reach whenever. Data is not directly found on the markup - This means that your markup is kept clean of any additional data, and only JavaScript sees it.

Cons

Latency - AJAX creates an HTTP request, and HTTP requests are carried over network and have network latencies. State - Data fetched via a separate HTTP request won't include any information from the HTTP request that fetched the HTML document. You may need this information (e.g., if the HTML document is generated in response to a form submission) and, if you do, will have to transfer it across somehow. If you have ruled out embedding the data in the page (which you have if you are using this technique) then that limits you to cookies/sessions which may be subject to race conditions.

实现示例

使用AJAX，你需要两个页面，一个是PHP生成输出的页面，另一个是JavaScript获得输出的页面:

get-data.php

/* Do some operation here, like talk to the database, the file-session
 * The world beyond, limbo, the city of shimmers, and Canada.
 *
 * AJAX generally uses strings, but you can output JSON, HTML and XML as well.
 * It all depends on the Content-type header that you send with your AJAX
 * request. */

echo json_encode(42); // In the end, you need to `echo` the result.
                      // All data should be `json_encode`-d.

                      // You can `json_encode` any value in PHP, arrays, strings,
                      // even objects.

Index.php(或任何实际页面的命名方式)

<!-- snip -->
<script>
    fetch("get-data.php")
        .then((response) => {
            if(!response.ok){ // Before parsing (i.e. decoding) the JSON data,
                              // check for any errors.
                // In case of an error, throw.
                throw new Error("Something went wrong!");
            }

            return response.json(); // Parse the JSON data.
        })
        .then((data) => {
             // This is where you handle what to do with the response.
             alert(data); // Will alert: 42
        })
        .catch((error) => {
             // This is where you handle errors.
        });
</script>
<!-- snip -->

上述两个文件的组合将在文件加载完成时提醒42。

更多的阅读材料

使用Fetch API 如何从异步调用返回响应?

2. 将数据回显到页面的某处，并使用JavaScript从DOM获取信息

这种方法不如AJAX好，但仍然有它的优点。PHP和JavaScript之间仍然是相对分离的在某种意义上，JavaScript中没有PHP。

Pros

快速——DOM操作通常很快，您可以相对快速地存储和访问大量数据。

Cons

Potentially Unsemantic Markup - Usually, what happens is that you use some sort of <input type=hidden> to store the information, because it's easier to get the information out of inputNode.value, but doing so means that you have a meaningless element in your HTML. HTML has the <meta> element for data about the document, and HTML 5 introduces data-* attributes for data specifically for reading with JavaScript that can be associated with particular elements. Dirties up the Source - Data that PHP generates is outputted directly to the HTML source, meaning that you get a bigger and less focused HTML source. Harder to get structured data - Structured data will have to be valid HTML, otherwise you'll have to escape and convert strings yourself. Tightly couples PHP to your data logic - Because PHP is used in presentation, you can't separate the two cleanly.

实现示例

因此，我们的想法是创建某种元素，它不会显示给用户，但对JavaScript是可见的。

index . php

<!-- snip -->
<div id="dom-target" style="display: none;">
    <?php
        $output = "42"; // Again, do some operation, get the output.
        echo htmlspecialchars($output); /* You have to escape because the result
                                           will not be valid HTML otherwise. */
    ?>
</div>
<script>
    var div = document.getElementById("dom-target");
    var myData = div.textContent;
</script>
<!-- snip -->

3.将数据直接回显到JavaScript

这可能是最容易理解的。

Pros

非常容易实现——它需要非常少的实现和理解。 不会弄脏源代码——变量直接输出到JavaScript，所以DOM不受影响。

Cons

将PHP与数据逻辑紧密结合——因为PHP用于表示，所以不能将两者清晰地分开。

实现示例

实现相对简单:

<!-- snip -->
<script>
    var data = <?php echo json_encode("42", JSON_HEX_TAG); ?>; // Don't forget the extra semicolon!
</script>
<!-- snip -->

好运！

只需使用以下方法之一。

<script type="text/javascript">
var js_variable  = '<?php echo $php_variable;?>';
<script>

OR

<script type="text/javascript">
    var js_variable = <?php echo json_encode($php_variable); ?>; 
</script>