• Support for namespaces
• Under the hood performance improvements
• Late static
  binding
• Lambda functionalities and closures
• Syntax additions: NOWDOC, limited GOTO,
  ternary short cut ''?.” and callStatic()
• Optional garbage collection for cyclic
  references
• Optional mysqlnd PHP native replacement for lib mysql
• Improved windows support
  including VC6 and VC9 binaries
• More consistent float rounding
• Deprecation notices are now
  handle via E_DEPRECATED (part of E_ALL) instead of the E_STRICT error level
• Several enhancements to
  enable more flexibility in php.ini ( and ini parsing in general )
• New bundled extensions: 
  ext/phar, ext/intl , ext/fileinfo, ext/sqlite3, ext/enchant
• Countless bug fixes and improvements to
  existing extensions in particular to: ext/openssl,ext/spl and ext/date

This release also drops several extensions and unifies usage of internal APIs. Users should be aware of the
following known backwards compatibility breaks:

• Parameter parsing API unification will cause
  some functions to behave more or less strict when it comes to type juggling
• Removed the following
  extensions: ext/mhash ( see ext/hash ), ext/msql, ext/pspell ( see ext/enchant ),ext/sybase ( see ext/sybase
  ct )
• Moved the following extensions to PECL: ext/ming ,
  ext/fbsql,ext/ncurses,ext/fdf
• Removed zend.ze1 compatibility mode