How to use dynamic value inside page configuration?

You can assign dynamic value to any parameters inside page configuration (*.pageconf file). You define your dynamic value using prefix 'fn::' for parameter value. As for example: fn::GetParam('context_id'). Any statement after prefix must can be evaluated and resulting a simple value.

Here is for your example:

<?xml version="1.0" encoding="UTF-8"?>
 <page name="HelpFile" id="HelpFile" Cache="true" CacheLifetime="0" CacheKeys="fn::GetParam('context_id')" >
  <processor PageSecurityCode="Page|HelpFile" HelpContext="LIGHTARMY-Applications.HelpFile" 
   sOnlyTemplateFileName="HelpFile.html" CanPublic="true" Authenticate="false"
   ValidateSystemConfiguration="false" >
   
   <PageSecurityCode>
    <Name>Help File</Name>
    <Description></Description>
    <HelpContext>LIGHTARMY-Applications.HelpFile</HelpContext>
   </PageSecurityCode>
   
   <TemplateProcessingScript><![CDATA[
    CIISONLINE_HelpFile::HelpFile_Show();
   ]]></TemplateProcessingScript>
   
   <ResponseProcessingScript><![CDATA[
    CIISONLINE_HelpFile::HelpFile_Print();
   ]]></ResponseProcessingScript>
   
   <IncludeFile>PageClass/HelpFile.php</IncludeFile>
   
  </processor>
 </page>