Not sure if you're looking at the source code on your computer, or GitHub, but the explanation and its source should be the same either way (see the comments above the function)
Thanks for such great tools.
I don't fully understand why people use is_numeric and not mean integer or double as in the explanation.
However I do feel a loss (small but still) in changing how to use it, as it does make the code readable and concise.
My two cents.