##First Use Existing Functions in WordPress Core
If you can: use existing functions included in WordPress core instead of writing your own. Only develop custom PHP functions when there is not an appropriate pre-existing function in WordPress core.
One benefit is you can use "log deprecated notices" to easily monitor functions that should be replaced. Another benefit is users can view the function documentation in the Codex and better understand what the plugin does even if they are not an experienced PHP developer.
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