With PHP 5.5 being stable and PHP 5.6 soon approaching, PHP 6.x would be the next logical major update to the language, but PHP 7.x is being considered. For those that haven't been into PHP development for a while, "PHP6" has been talked about for years -- going back almost a decade -- but PHP 6.0 never materialized and ultimately its focus ended several years ago. Many features ended up being added to PHP 5.x revisions from PHP 5.3 and forward while the focus on producing PHP 6.0 ended officially in 2010, four years after PHP 6.0 was originally planned for a release.
Due to many sites, books, and other resources having talked about PHP6 plans, which then failed, developers are wondering whether the next radical release of PHP should be called PHP 7 to avoid confusion with previous PHP 6 communication. Basically, they're trying to cause less confusion for new PHP developers by skipping a version.
PHP developers started voting yesterday on the PHP6 vs. PHP7 naming, but that vote was called off for the time being. More information on the PHP next-generation naming can be found via this PHP RFC.