The reasoning is simple: The prefix "_" is usually either used for easier distinction of instance variables from other, i.e. local variables, instead of always using "this." (often seen in C#), or it is used to signal that a member is not meant to be accessed from outside of the class when the language does not have visibility modifiers (PHP4).
Since you always have to use "$this->" in PHP5+ when accessing instance members and there are visibility modifiers, the "_" is largely superfluous and just makes the verbose OO code even more verbose.
Maybe the following find/replace steps will do the job almost completely:
"private $_" => "private $"
"protected $_" => "protected $"
"$this->_" => "$this->"