The Work is in progress for a new PHP version which is named PHP 7. Before that it was a discussing matter that the next version of Php is 6 or PHP 7 but  as the PHP 6 unicode experiment has not released so after vote the PHP 7 has been chosen to release.

PHP 7 gets its base from an experimental PHP branch that was originally named phpng (PHP next generation), which aims to optimize PHP performance by refactoring the Zend Engine while retaining near-complete language compatibility.  As of 14 July 2014, WordPress-based benchmarks, which serve as the main benchmark suite for the phpng project, show an almost 100% increase in performance.
And the Changes from phpng are also expected to make it easier to improve performance in the future, as it have  more compact data structures and other changes are seen as better suited for a successful migration to a just-in-time (JIT) compiler. Because of the significant changes, this reworked Zend Engine will be called Zend Engine 3, succeeding the Zend Engine 2 used in PHP 5.

As there is major changes so it would have to go for a new major version of PHP rather than the minor release 5.x.
Major versions of PHP are allowed to break code backwards-compatibility, and so PHP 7 presented an opportunity to make other improvements beyond phpng that require backwards-compatibility breaks. In particular, the following backwards-compatibility breaks were made

The major changes in PHP 7 are as follows:
1. Many "fatal" or "recoverable"-level legacy PHP "errors" were replaced with modern object-oriented exceptions
2.The syntax for variable dereferencing was reworked to be more internally consistent and complete, allowing the use of ->, [], (), {}, and :: operators with arbitrary meaningful left-hand-side expressions
3. Support for legacy PHP 4-style constructor methods was deprecated
4. The behaviour of the foreach statement was changed to be more predictable
5. Constructors for the few classes built-in to PHP which returned null upon failure were changed to throw an exception instead, for consistency
6. Several unmaintained or deprecated SAPIs and extensions were removed from the PHP core, most notably the legacy mysql extension
7. The behaviour of the list() operator was changed to remove support for strings
8. Support for legacy ASP-style PHP code delimeters (<% and %>, <script language=php> and </script>) was removed
9. An oversight allowing a switch statement to have multiple default clauses was fixed
10. Support for hexadecimal number support in some implicit conversions from strings to number types was removed
11. The left-shift and right-shift operators were changed to behave more consistently across platforms
12. Conversions between integers and floating point numbers were tightened and made more consistent across platforms
13. PHP 7 will also include new language features. Most notably, it will introduce return type declarations, which will complement its existing parameter type declarations, and support for the scalar types (integer, float, string and boolean) in parameter and return type declarations.

Thanks!!