Add initial support for PHP8

Testcase director_overload2 is failing, but the rest of the testsuite
passes.

Doc/Manual/Php.html

@@ -51,12 +51,12 @@
 
 <p>
 In this chapter, we discuss SWIG's support of PHP.  SWIG currently supports
-generating wrappers for PHP7.  Support for PHP5 was removed in SWIG 4.0.0
-and support for PHP4 was removed in SWIG 1.3.37.
+generating wrappers for PHP7 and PHP8.  Support for PHP5 was removed in SWIG
+4.0.0 and support for PHP4 was removed in SWIG 1.3.37.
 </p>
 
 <p>
-Currently any PHP7 release should work.
+Currently any PHP7 or PHP8 release should work.
 </p>
 
 <p>

Doc/Manual/Preprocessor.html

@@ -123,7 +123,7 @@ SWIGOCAML                       Defined when using OCaml
 SWIGOCTAVE                      Defined when using Octave
 SWIGPERL                        Defined when using Perl
 SWIGPHP                         Defined when using PHP (any version)
-SWIGPHP7                        Defined when using PHP7
+SWIGPHP7                        Defined when using PHP 7 or later
 SWIGPYTHON                      Defined when using Python
 SWIGR                           Defined when using R
 SWIGRUBY                        Defined when using Ruby

Doc/Manual/SWIG.html

@@ -127,7 +127,7 @@ Supported Target Language Options
      -lua            - Generate Lua wrappers
      -octave         - Generate Octave wrappers
      -perl5          - Generate Perl 5 wrappers
-     -php7           - Generate PHP 7 wrappers
+     -php7           - Generate PHP 7 or later wrappers
      -python         - Generate Python wrappers
      -r              - Generate R (aka GNU S) wrappers
      -ruby           - Generate Ruby wrappers