Personally, this can go either way. Some programmers prefer the CamelCase way of writing, others prefer underscore.. So asking a question like this posses it's irrelevance as each programmer will have their own different opinion.. Personally, I have the habit of doing both depending on the situation..
You have joined a programming team and have joined an on-going project, you notice that an example variable is set out:
$TheString = "This is a camelcase example";
But you have a preferred style of:
$The_String = "This is not a camelcase example";
in a situation like this, it would be best to keep to camelcase.
You wish to start a project solo.
In this sort of case.. It just goes down to what you feel comfortable with, bare in mind bad practices in terms of:
$TheString = "This is a sample String";
$The_String = "This is another sample string within the same project";
This can easily cause unexpected output.. Just bare that in mind.