Add a phase_name() macro.

embed.fnc

@@ -3699,4 +3699,6 @@ XEop	|STRLEN*|dup_warnings	|NULLOK STRLEN* warnings
 Amd	|void	|CopFILEGV_set	|NN COP * c|NN GV * gv
 #endif
 
+Amd|char *const|phase_name|enum perl_phase
+
 : ex: set ts=8 sts=4 sw=4 noet:

perl.h

@@ -5431,6 +5431,24 @@ EXTCONST char *const PL_phase_names[] = {
 EXTCONST char *const PL_phase_names[];
 #endif
 
+/*
+=for apidoc_section $utility
+
+=for apidoc Amd|char *const|phase_name|enum perl_phase
+
+Returns the given phase's name as a NUL-terminated string.
+
+For example, to print a stack trace that includes the current
+interpreter phase you might do:
+
+    const char* phase_name = phase_name(PL_phase);
+    mess("This is weird. (Perl phase: %s)", phase_name);
+
+=cut
+*/
+
+#define phase_name(phase) (PL_phase_names[phase])
+
 #ifndef PERL_CORE
 /* Do not use this macro. It only exists for extensions that rely on PL_dirty
  * instead of using the newer PL_phase, which provides everything PL_dirty

pod/perldelta.pod

@@ -346,9 +346,8 @@ well.
 
 =over 4
 
-=item *
-
-XXX
+=item * A new phase_name() interface provides access to the name
+for each interpreter phase (i.e., PL_phase value).
 
 =back