Snake-case vs camel-case
I like to use snake-case over camel-case for all variable and function names.
If variable names are made of multiple words, and words are usually separated by spaces, then swapping the space for an underscore seems better than joining them together then trying to figure out if you drop the case on an acronym.
XML_to_JSON > XMLToJSON xml_to_json > XmlToJson
I follow this same rule when hyphens are allowed, such as file names and CSS classes.