Call PHP variable in CF7

Hi,

I’ve stumbled upon a rather unique issue.

In my theme’s header.php, I have the following code.
$ConvCheckboxRandomNo = rand(10000,90000);

I need to call this variable in my contact form.
Something like this:
<input type=”hidden” name=”MyName” value=”$ConvCheckboxRandomNo”>
AND in Javascript
<script>alert(‘$ConvCheckboxRandomNo’);</script>”;

Can you tell me how this can be done. I tried adding it simply and expectedly it did not work. What is the correct way to do this?

Thanks!