False positive for undefined variable in preg_replace about phpcs-variable-analysis HOT 1 CLOSED

That's a very good observation, thanks! For later reference, the documentation for the second argument of preg_replace() reads:

replacement may contain references of the form \n or $n, with the latter form being the preferred one.

This could be handled as a special case: ignore undefined variables between the parenthesis of preg_replace() if the names of those variables are entirely numeric.

On the other hand,

A valid variable name starts with a letter or underscore, followed by any number of letters, numbers, or underscores

So actual PHP variables cannot ever start with a number. Perhaps we could solve this simply by always ignoring variables that start with a number. If they exist and are illegal, some other sniff should detect them first. I think I like this solution better because it's simpler to implement and actually should decrease the amount of processing this sniff has to do.

from phpcs-variable-analysis.