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how do I know which template variable, for instance, U_SITE_HOME or U_SEARCH comes from where? Is it possible with phpstorm to get to its implemention by marking it?
Common ones are found at the bottom /includes/functions.php , specific ones only relative to that page are found at the bottom of that file, e.g. viewtopic.php. There are some other scattered throughout the files and best found search for it using Notepad++ or something similar which can search all files.
Note that ones prefixed with a L_ are from the language files and don't include the L_ when searching.
“Results! Why, man, I have gotten a lot of results! I have found several thousand things that won’t work.”