Why You Should Not Modify Core OpenCart Files

Posted by kevin on August 28, 2025

You really should not modify any of the core files that are installed by default with the OpenCart installation. This sounds like advice against taking advantage of one the best parts of dealing with open-source software, but with the OCMod system introduced in OpenCart 2, or the vQmod system developed separately, this is not incompatible with modifying the functionality of your site, but rather the means of keeping the site's integrity over OpenCart version updates, maintaining extension compatibility, and keeping changes easily reversible.

1. Version Updates

 By keeping changes contained to modifications, the core files are left untouched and upgrading to a later version can, in simpler cases such as having no or few extensions, be as simple as re-uploading the OpenCart core files over the site's current files. If any core files were modified, this would undo any changes made, and require additional work to copy over every change into the upgraded version.

2. Extension Compatibility

 most extensions use modification systems like OCMod or vQmod to function. Given that these work by finding lines in certain target files and performing operations such as replacing or adding lines of code, any modification to a core file has a chance to prevent an existing extension from working by causing it to be unable to find the point where it should modify the functionality of the site, due to a core file change causing a line it searches for to be changed.

3. Reversible Changes

When you modify core files, reverting the change made would require finding all files modified due to the specific functionality you added, and replacing then with the original file, while copying  any other modification to those files that you don't want removed. With an OCMod, related changes for a single purpose can be kept as a single OCMod, and disabling it is as simple as uninstalling the modification, without needing to worry about other modifications to the same files.

Given these Considerations, it is almost always better to not modify the OpenCart core files, and instead use OCMods to modify the functionality of a site. An useful tool for building your own OCMods is the official documentation:

https://github.com/opencart/opencart/wiki/modification-system