PHP语法分析？

我编辑了Q，因为没有答案

我是PHP的初学者，正在编写CMS模板引擎。这个想法很简单——我会在herdoc字符串中预加载HTML片段，然后在流控件要求时调用它们

问题是herdoc字符串的行为很奇怪。以下是一个例子：

$ERROR_MESSAGE = "default";
$tst = <<<EOF
<div id="error-message">$ERROR_MESSAGE</div>
EOF;

function splice_message_output()
    {
    global $ERROR_MESSAGE;
    global $tst;

    if (isset($ERROR_MESSAGE))
        {
        echo <<<EOF
<div id="error-message">$ERROR_MESSAGE</div>
EOF;
        echo $tst;
        }
    }

输出为：

<div id="error-message">This is the errormessage text</div>
<div id="error-message">default</div>

这怎么可能？我已经在下面的代码中清楚地定义了调用的strign，herdoc代码段返回刚刚在上面设置的字符串，另一个函数返回刚刚在下面设置的字符串。

您只定义一次$tst，其中包含消息“default”。函数下面的代码设置$ERROR\u MESSAGE=$\u GET[MSG]；仅当设置了$_GET[MSGTYPE]且等于“ERR”时。因此，它设置变量，打印嵌入的HEREDOC字符串，然后打印未更改的$tst。这里的埃雷多克一点也不奇怪

---

我认为您感到困惑的是，您认为herdoc字符串将始终包含对$ERROR\u消息的引用。事实并非如此。在字符串中使用$ERROR\u MESSAGE之类的变量时，该变量将被其内容替换，这与使用$tst=消息时的情况完全相同：$ERROR\u MESSAGE；。之后，$tst的值将是“消息是：默认值”。

如前所述，在更改$ERROR\u消息的值之前，您已经声明了$tst的值。你对heredoc的理解是这里的问题。当你将一个变量插入到一个heredoc或者任何类型的字符串中时，这并不意味着你要插入这个变量本身或者变量的引用。而是将该变量的值插入到herdeoc中。例如，如果你有以下变量

$var1 = "inserted var";
$var2 =
<<<VAR2
My $var1 goes here.
VAR2;

Heredoc函数与其他字符串一样。不同之处在于，在heredoc中，您避免了在“和”和/或任何进一步的“逃离地狱”之间逃跑和交换的需要。总的来说，herdoc只是为了让代码更易于阅读和维护

<?Php
echo "<button onclick='alert(\"double-quote\")'>double-quote</button>";
echo '<button onclick="alert('."'single-quote'".')">single-quote</button>';
echo
<<<HEREDOCSAMPLE
<button onclick='alert("heredoc")'>heredoc</button>
HEREDOCSAMPLE;
?>

现在在您的代码示例中：

$ERROR_MESSAGE = "default";
$tst =
<<<EOF
<div id="error-message">$ERROR_MESSAGE</div>
EOF;
//THIS WOULD MAKE THE CURRENT VALUE OF $tst as:
//<div id="error-message">default</div>

function splice_message_output()
{   global $ERROR_MESSAGE;
    global $tst;

    if (isset($ERROR_MESSAGE))
    // I DON'T THINK THIS VALIDATION MAKES SENSE SINCE $ERROR_MESSAGE
    // WOULD ALWAYS BE DECLARED
    {   echo <<<EOF
<div id="error-message">$ERROR_MESSAGE</div>
EOF;
        echo $tst;
    }
}

if (isset($_GET["MSGTYPE"]))
{   if ($_GET["MSGTYPE"] == "ERR")
    {   $ERROR_MESSAGE = $_GET["MSG"];
        // THE ONLY VARIABLE THAT CHANGED HERE IS $ERROR MESSAGE
        // $tst DID NOT CHANGE AT ALL
    }
}

function splice_message_output()
// RE-DECLARED FUNCTION? OR PERHAPS:
splice_message_output();
// AT THIS POINT ONLY $ERROR_MESSAGE HAS CHANGED ITS VALUE.

现在，如果我提议对该代码进行修订，则如下所示：

$ERROR_MESSAGE = "default";
function splice_message_output()
{   global $ERROR_MESSAGE;
    global $tst;

    if (isset($_GET["MSG"]))
    {   echo <<<EOF
<div id="error-message">$ERROR_MESSAGE</div>
EOF;
        $tst = <<<EOF
<div id="error-message">$ERROR_MESSAGE</div>
EOF;
        // NOW HERE'S A TRICKY ONE. SINCE YOU DECLARED $tst AS A
        // GLOBAL VARIABLE WITHIN THIS FUNCTION, DOING AN if(isset($tst))
        // OUTSIDE THIS FUNCTION AFTER CALLING THIS FUNCTION WOULD
        // BE EQUAL TO 'TRUE'
        echo $tst;
    }
}

if (isset($_GET["MSGTYPE"]))
{   if ($_GET["MSGTYPE"] == "ERR")
    {   $ERROR_MESSAGE = $_GET["MSG"];
        // THE ONLY VARIABLE THAT CHANGED HERE IS $ERROR MESSAGE
        // $tst DID NOT CHANGE AT ALL
    }
}

// THE VARIABLE $tst DOESN'T EXIST AT THIS POINT BUT
splice_message_output(); // ONCE THIS FUNCTION IS CALLED
// $tst WOULD HAVE ALREADY EXISTED

在更改$ERROR_MEGGAGE的值之前，将先评估$tst的值。因此，在将$ERROR\u消息用于$tst的herdeoc语句之前，只需检查您的条件并对其值进行任何更改

<?Php
echo "<button onclick='alert(\"double-quote\")'>double-quote</button>";
echo '<button onclick="alert('."'single-quote'".')">single-quote</button>';
echo
<<<HEREDOCSAMPLE
<button onclick='alert("heredoc")'>heredoc</button>
HEREDOCSAMPLE;
?>

$this = "example";
echo
<<<'NOWDOCSAMPLE'
$this variable would not be treated as a $variable.
NOWDOCSAMPLE;

$this variable would not be treated as a $variable.

$ERROR_MESSAGE = "default";
$tst =
<<<EOF
<div id="error-message">$ERROR_MESSAGE</div>
EOF;
//THIS WOULD MAKE THE CURRENT VALUE OF $tst as:
//<div id="error-message">default</div>

function splice_message_output()
{   global $ERROR_MESSAGE;
    global $tst;

    if (isset($ERROR_MESSAGE))
    // I DON'T THINK THIS VALIDATION MAKES SENSE SINCE $ERROR_MESSAGE
    // WOULD ALWAYS BE DECLARED
    {   echo <<<EOF
<div id="error-message">$ERROR_MESSAGE</div>
EOF;
        echo $tst;
    }
}

if (isset($_GET["MSGTYPE"]))
{   if ($_GET["MSGTYPE"] == "ERR")
    {   $ERROR_MESSAGE = $_GET["MSG"];
        // THE ONLY VARIABLE THAT CHANGED HERE IS $ERROR MESSAGE
        // $tst DID NOT CHANGE AT ALL
    }
}

function splice_message_output()
// RE-DECLARED FUNCTION? OR PERHAPS:
splice_message_output();
// AT THIS POINT ONLY $ERROR_MESSAGE HAS CHANGED ITS VALUE.

$ERROR_MESSAGE = "default";
function splice_message_output()
{   global $ERROR_MESSAGE;
    global $tst;

    if (isset($_GET["MSG"]))
    {   echo <<<EOF
<div id="error-message">$ERROR_MESSAGE</div>
EOF;
        $tst = <<<EOF
<div id="error-message">$ERROR_MESSAGE</div>
EOF;
        // NOW HERE'S A TRICKY ONE. SINCE YOU DECLARED $tst AS A
        // GLOBAL VARIABLE WITHIN THIS FUNCTION, DOING AN if(isset($tst))
        // OUTSIDE THIS FUNCTION AFTER CALLING THIS FUNCTION WOULD
        // BE EQUAL TO 'TRUE'
        echo $tst;
    }
}

if (isset($_GET["MSGTYPE"]))
{   if ($_GET["MSGTYPE"] == "ERR")
    {   $ERROR_MESSAGE = $_GET["MSG"];
        // THE ONLY VARIABLE THAT CHANGED HERE IS $ERROR MESSAGE
        // $tst DID NOT CHANGE AT ALL
    }
}

// THE VARIABLE $tst DOESN'T EXIST AT THIS POINT BUT
splice_message_output(); // ONCE THIS FUNCTION IS CALLED
// $tst WOULD HAVE ALREADY EXISTED