Which PHP 5.X version should be considered as Required

This is hard point of discussion. 
Each PHP major version 5.4, 5.5, 5.6 has its own nice things.

Here is short description of latest 3 major versions changelog:
5.4

The key features of PHP 5.4.0 include:

• New language syntax including Traits, shortened array syntax and more
  
  
• Improved performance and reduced memory consumption
  
  
• Support for multibyte languages now available in all builds of PHP at the flip of a runtime switch
  
  
• Built-in webserver in CLI mode to simplify development workflows and testing
  
  
• Cleaner code base thanks to the removal of multiple deprecated language features
  
  
• Many more improvements and fixes
  

Changes that affect compatibility:

• Register globals, magic quotes and safe mode were removed
  
  
• The break/continue $var syntax was removed
  
  
• The ini option allow_call_time_pass_reference was removed
  
  
• The PHP default_charset is now "UTF-8" within the distributed php.ini files, but still defaults to ""
  

5.5:

The key features of PHP 5.5.0 include:

• Added generators and coroutines.
  
  
• Added the finally keyword.
  
  
• Added a simplified password hashing API.
  
  
• Added support for constant array/string dereferencing.
  
  
• Added scalar class name resolution via ::class.
  
  
• Added support for using empty() on the result of function calls and other expressions.
  
  
• Added support for non-scalar Iterator keys in foreach.
  
  
• Added support for list() constructs in foreach statements.
  
  
• Added the Zend OPcache extension for opcode caching.
  
  
• The GD library has been upgraded to version 2.1 adding new functions and improving existing functionality.
  
  
• A lot more improvements and fixes.
  

Changes that affect compatibility:

• PHP logo GUIDs have been removed.
  
  
• Windows XP and 2003 support dropped.
  
  
• Case insensitivity is no longer locale specific. All case insensitive matching for function, class and constant names is now performed in a locale independent manner according to ASCII rules.
  

5.6:

The main features of PHP 5.6.0 include:

• Constant scalar expressions.
  
• Variadic functions and argument unpacking using the ... operator.
  
• Exponentiation using the ** operator.
  
• Function and constant importing with the use keyword.
  
• phpdbg as an interactive integrated debugger SAPI.
  
• php://input is now reusable, and $HTTP_RAW_POST_DATA is deprecated.
  
  
• GMP objects now support operator overloading.
  
• File uploads larger than 2 gigabytes in size are now accepted.
  
  

PHP 5.6.0 also introduces changes that affect compatibility:

• Array keys won't be overwritten when defining an array as a property of a class via an array literal.
  
  
• json_decode() is more strict in JSON syntax parsing.
  
  
• Stream wrappers now verify peer certificates and host names by default when using SSL/TLS.
  
  
• GMP resources are now objects.
  
  
• Mcrypt functions now require valid keys and IVs.
  

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