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, bear 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 bear that in mind.