"Interactive mode enabled" means that it is reading from stdin but does not give you a full shell. The prompt you're expecting comes after an "Interactive shell" message.

Did you build with readline support? I don't see a --with-readline in your configure command. Or anything else, for that matter.
And just in case, what commit did you build from? Was it HEAD within the last ~16 hours?

No, I did not --with-readline my ./configure command.

Yes, this was from HEAD on the master branch on Github: commit hash c945c399aa8051ae24c727d73c1ea0989f4d14d7

I've rebuilt it --with-readline and now I'm getting a weird error I don't understand, but it looks unrelated:

/usr/bin/ld: ext/reflection/php_reflection.o: in function `zim_ReflectionFiber___construct':
/home/scott/tmp/php-src/ext/reflection/php_reflection.c:6770: undefined reference to `zend_ce_fiber'
/usr/bin/ld: ext/reflection/php_reflection.o: in function `instanceof_function':
/home/scott/tmp/php-src/Zend/zend_operators.h:71: undefined reference to `zend_ce_fiber'
/usr/bin/ld: ext/reflection/php_reflection.o: in function `zim_ReflectionFiber___construct':
/home/scott/tmp/php-src/ext/reflection/php_reflection.c:6779: undefined reference to `zend_ce_fiber'
/usr/bin/ld: Zend/zend_execute_API.o: in function `init_executor':
/home/scott/tmp/php-src/Zend/zend_execute_API.c:195: undefined reference to `zend_fiber_init'
/usr/bin/ld: Zend/zend_default_classes.o: in function `zend_register_default_classes':
/home/scott/tmp/php-src/Zend/zend_default_classes.c:42: undefined reference to `zend_register_fiber_ce'
collect2: error: ld returned 1 exit status
make: *** [Makefile:274: sapi/cli/php] Error 1

> The prompt you're expecting comes after an "Interactive shell" message.

Interestingly, in my copy of PHP 8.0.x (provided by the ondrej packages for Ubuntu), I see the following prompt:

$ php8.0 -a
Interactive mode enabled

php > 

...Is this "Interactive mode enabled" / "Interactive shell" verbage something new to PHP 8.1? If that discrepancy isn't new, then readline might be a red herring, because it's working as expected when that prompt is displayed.

> I've rebuilt it --with-readline and now I'm getting a weird error I don't
> understand, but it looks unrelated:

It sure does look unrelated, and c945c39 just built successfully for me. Did you reuse the same build directory from before? Did you make clean/git clean it before the second build?

> Interestingly, in my copy of PHP 8.0.x (provided by the ondrej packages for
> Ubuntu), I see the following prompt:

The relevant php_cli.c source [1] has changed some with 8.1, that is true:

if (interactive) {
	if (cli_shell_callbacks.cli_shell_run) {
		printf("Interactive shell\n\n");
	} else {
		printf("Interactive mode enabled\n\n");
		/* Treat as non-interactive apart from the stdin input */
		interactive = false;
	}

With 8.0 and before, there were still two messages displayed [2]: "shell" if you had specifically libreadline or libedit support and "mode" if not. What about building the same 8.0.x version yourself?


[1] https://github.com/php/php-src/blob/c945c39/sapi/cli/php_cli.c#L876
[2] https://github.com/php/php-src/blob/PHP-8.0.6/sapi/cli/php_cli.c#L867

To clarify how this works: If you have readline enabled, then "Interactive shell" is printed, and you get a normal interactive shell, where you can enter something and press enter to run it. If readline is not enabled, then "Interactive mode enabled" is printed, and PHP just reads from stdin. You can use this mode by entering something and then using Ctrl+D to close stdin.

This is all pretty confusing, and the -a mode without readline seems pretty much useless to me. Maybe we should just completely disable -a if readline is not available?

> With 8.0 and before, there were still two messages displayed
> [2]: "shell" if you had specifically libreadline or libedit
> support and "mode" if not.

The point is that since yesterday, readline support was determined
during build time, so when building ext/readline separately (like
it's usually done for Linux distros), you always got the
"Interactive mode" message, even when it actually may be the
interactive shell.

s/since yesterday/before yesterday