Variable interpolation after sending through HTML form.

Two pages.

// Step1.html
<form action="step2.php" method="POST">
<textarea cols="50" rows="5" name="HTMLContent"></textarea>
<input type="Submit" value="Submit">
</form>

// Step2.php
<?PHP
$InterpolationVariable = "Interpolation worked";
$PreviousContent = stripslashes($HTTP_POST_VARS[HTMLContent]);

echo $PreviousContent;
?>

// Problem
When I put normal HTML in the HTMLContent textarea of step1.html, it outputs just fine. However when I add $InterpolationVariable to the HTML in the HTMLContent textarea of step1.php it outputs the literal $InterpolationVariable within the html.

// Example of TextArea Input
<STRONG>This part works.</STRONG><br>
<STRONG>The $InterpolationVariable</STRONG><br>

// Example of Step2.php Output
This part works(IN BOLD)
The $InterpolationVariable(IN BOLD)

// Conclusion
I'm not sure if this is really called variable interpolation so please forgive me if it's not. Is some function like stripslashes(); that will input the variables into the submitted content?

Thanks in advance,
Charles

well you can't use a variable that hasn't been defined yet.. for something like that you'd probably want to go with the str_replace function.. for example, in step 2 use:

instead of:

and in the textarea just type "InterpolationVariable" where you want it replaced instead of using the variable format. i'm sure there's other ways of accomplishing the same thing if you don't care for this particular method

The above code would treat the data put into the form as PHP, and so substitute any variables for their value. It would also run any functions. To put run functions or use variables from the TEXTAREA, you would treat it like a normal PHP script:

<STRONG>Hi There</STRONG><BR>
<?php echo("$InterpolationVariable"); ?>


This would produce, in Step2.php the following:

Hi there (Bold)
Interpolation worked

WARNING: This would give access to ANYONE to run ANYTHING (in PHP) on your server.