Make callback unit test pass for PHP backend.

Deprecated %callback(1) doesn't work with PHP, use "%s" to give the same name
to the callback as to the C function explicitly instead.

Examples/test-suite/php/Makefile.in

@@ -10,6 +10,7 @@ top_srcdir   = @top_srcdir@
 top_builddir = @top_builddir@
 
 CPP_TEST_CASES += \
+	callback \
 	php_iterator \
 	php_namewarn_rename \