So, I\'m making a site about WWI as a school assignment, and I want this to appear in every document:
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You could consider one of these popular template engines :
Smarty (becoming outdated)
Latte (used mostly by the Nette community)
Twig (used mostly by the Symfony community)
Mustache (official implementations of this templating engine exist in more than two dozen proramming/scripting languages)
I tend to favor Mustache, mostly because it has an official JS version, an official Ruby version, an official Java version, etc. It allows you to use the same templates frontend and backend, which is very useful for widgets that are first rendered in background and rerendered in foreground at updates.