[PHP7] Make default C++ extension .cxx

This is SWIG's default for every language except PHP, and now is a
good time to make this change so that once we drop PHP5 support, we'll
have the same default everywhere.  It's easy to override the default
with -cppext cxx to get the PHP5 behaviour.

Doc/Manual/Php.html

@@ -106,7 +106,12 @@ also contain PHP class wrappers.
 <p>
 SWIG can generate PHP extensions from C++ libraries as well when
 given the <tt>-c++</tt> option.  The support for C++ is discussed in
-more detail in <a href="#Php_nn2_6">section 27.2.6</a>.
+more detail in <a href="#Php_nn2_6">section 27.2.6</a>.  The generated
+C++ wrapper will be called example_wrap.cpp (for PHP5) or
+example_wrap.cxx (for PHP7 where the default has been changed to align
+with SWIG's default for every other language).  You can specify a
+different extension for the C++ wrapper using <tt>-cppext</tt> -
+e.g. if you want example_wrap.cc use <tt>-cppext cc</tt>.
 </p>
 
 <p>

Doc/Manual/SWIG.html

@@ -145,7 +145,7 @@ can be obtained by typing <tt>swig -help</tt> or <tt>swig
 
 -c++                  Enable C++ parsing
 -cppext <em>ext</em>           Change file extension of C++ generated files to <em>ext</em>
-                      (default is cxx, except for PHP which uses cpp)
+                      (default is cxx, except for PHP5 which uses cpp)
 -D<em>symbol</em>              Define a preprocessor symbol
 -Fstandard            Display error/warning messages in commonly used format
 -Fmicrosoft           Display error/warning messages in Microsoft format