What do you mean? This is the same message shown for any non-existent extension.

Yes, right, and rightly so. If you give PHP the directive to try a null string for the directive it resolves it as a relative pathname. PHP does not stop to load here. Don't provide it if you don't mean it.

Did you read the original issue? The aim is to have something like:

{if hasEnv("FOO")}
extension=foo.so
{/if}

Yes I did, this is the equivalent of

.PHONY : conditional
conditional :
# If people set these on the command-line, use them
PHP ?= php
PHPFLAGS ?=
ifdef FOO
 PHPFLAGS += --define extension=foo.so
endif
conditional :
	$(PHP) $(PHPFLAGS) -r 'printf("Hello, world!\n");'

Biased towards an in-work-tree invocation, and it shows more clearly what the extension directive stands for: load the extension and with it, load the extensions' directives. This makes the two directives, extension and zend_extension, special.

The equivalent at the entry-point of a system image packaged FROM the Docker library is much shorter and works since PHP 5.4:

if [ "$FOO" ]
then
 PHP_INI_SCAN_DIR="$PHP_INI_DIR/cfg-foo:$PHP_INI_SCAN_DIR"
fi

Note the colon ":" in the listing; the empty entry stands for the compile time value (ex. "$PHP_INI_DIR/conf.d" in a PHP container from the library). You may want to flip the order (append instead of prepend) or entirely set/replace.

Table: Environment Parameter at the Entry-Point affecting PHP Resource Configuration and Scanning for INI-Files

To complete the example:

$ cat "$PHP_INI_DIR/cfg-foo/foo.ini"
extension = foo.so

The OPs' configuration directive, now conditionally parsed based on the FOO environment parameter.

Problem solved. Otherwise, there is /tmp in a read-only container.

Nevertheless, to be clear: Our problem is not a read-only file-system here, from a systems engineering perspective we were longtime coming from the standpoint that files in sysconfigdir are not changed by the user nor the program. This is also the reason why the OP's-known solution actually is technically wrong when packaging the system image, and we should avoid any potential x/y problems here.

without having to invent new syntax. Tim suggested a simple approach of ignoring empty extension directives. As mentioned on the list, if the fear is that we might accidentally pass empty environment variables without warning, we could use a different sentinel value. Inventing a new if conditional would be significantly more involved, though also solve more problems.

Yes, right, and if we still want to patch the configuration loading I'd recommend to add two new directives:

1. extension_ifdef
2. zend_extension_ifdef

they only delegate to their sibling if the value on the right-hand side represents a non-empty string; that is:

1. not "" and
2. not false and
3. not null

This helps to clearly differentiate between their ground form and the intended use, that is to achieve the command-line directive semantics as outlined in the Makefile listing for PHPFLAGS above.

There is also no need to find a sentinel value; I think empty is the best value for what we want to express here, especially as we have to map environment parameter effective and efficiently.
And the two new proposed directives are speaking a clear language. As those are new directives, they should work on the command line, too.

Other directives should not be affected: we use extension and zend_extension to shortcut the effectiveness of the extensions configuration directives (if it could be loaded) so we move the condition to the place where it belongs in the logical order of the abstractions.

flowchart LR
 A((start))
 A --> C
 A --> d1
 A --> d2
 d1(extension_ifdef) ---> B
 B{defined?} -- no --> C
 C((stop))
 B -- yes --> d2
 d2(extension)
 d2 --> C

Furthermore, and additionally I propose we should add another long option and an ini-file counterpart.

The --define-env <name>=<envvar> long option that is similar to --define <name>=<value> and set the directive <name> where <envvar> is the name of an environment variable from which to retrieve the value. Unlike -d there is no shortcut for directly setting the value to an empty string, instead the environment variable itself must be set to the empty string. It is an error if the <envvar> does not exist in the environment. (copied from git(1))

[PHP]
example_directive:env = ENVVAR

The right hand side evaluates to the name of an environment variable. It is an error if it does not exist in the environment.

Consequences of an error in these instances: The runtime stops loading the configuration and exits with a non-zero exit status not larger than 254, including.

To share the benefit of the new error handling within the configuration, I'd further-furthermore and additionally suggest to have ':?'/'?' semantics with the environment parameter expansion in php-ini-files:

example_directive = ${ENVVAR?}

Given ENVVAR does not exist in the environment, it is an error with the same consequences as already outlined.

example_directive = ${ENVVAR:?}

Given ENVVAR does not exist in the environment or is null, it is an error with the same consequences as already outlined.