Yes, it is great idea and I planned to add this feature in future.
Also I planned to add analog of eval and other callback and user-defined functions.

There is not a lot of work, but there is the difficulty of choosing from a variety of possible variants.
Operator "Include" can be used like native and transclusion can be released as:

$something = call_user_func_array( "Template:Get_something", [ 'foo' => 'bar', 'baz' ] );

I think this is the best variant that fits php and wiki.
Other variants:

$something = "Template:Get_something"( [ 'foo' => 'bar', 'baz' ] );

$func = function "Template:Get_something"; 
$something = $func( [ 'foo' => 'bar', 'baz' ] ); // or $func( 'bar', 'baz' ) but it doesn't support named arguments.

$something = call_template( "Template:Get_something", [ 'foo' => 'bar', 'baz' ] );
$something = transclude( "Template:Get_something", [ 'foo' => 'bar', 'baz' ] );
etc...

What do you think about this?

Change 209703 had a related patch set uploaded (by Pastakhov):
Allow transclusion of templates inside PhpTags (v 5.1.0) Hook release 8

https://gerrit.wikimedia.org/r/209703

I've been thinking about it, but not sure what's best. Some thoughts on various options:

If support is later added for defining user functions the same way as in PHP inside phptags blocks (function name( ... ) { ... }), then it's possible for page names to conflict with function names. It may not be a big problem: the user could avoid it for example by naming functions with a leading '_', which is never in a page name.

Using a keyword, like the example $func = function "Template:Get_something"; could also be an option to avoid conflict. Perhaps it would also be good to have for flexibility. When using a string which could be the name of either a page or a user function, another way to avoid conflict is to have both call_user_func_array and an operator like transclude:

• If a string is given to call_user_func_array, it could first check for a user function definition before trying to use the string as a page name.
• transclude could always treat a string as a page name.

Maybe this is not relevant right now. And I don't know if you plan to eventually support PHP-style user function definitions.

About passing arguments for transclusions: passing an array seems the simplest way to support named arguments. Another way could be to support strings similar to those used in MediaWiki parser functions, "name=value". Then arrays would become the way to pass "raw" arguments, and strings would become a way to pass arguments filtered like MediaWiki does. (If other types are also supported, they might be unnamed raw arguments.) Maybe this would also be worth supporting, though I think the array option gives the most flexibility and would be the simplest to add. If some alternative is supported, there could also be better ideas for it.

Change 212750 had a related patch set uploaded (by Pastakhov):
add transclude function (v 3.5.0) requere PhpTags >= 5.1.0

https://gerrit.wikimedia.org/r/212750

Change 209703 merged by jenkins-bot:
Allow transclusion of templates inside PhpTags (v 5.1.0) Hook release 8 T99997

https://gerrit.wikimedia.org/r/209703

Change 212750 merged by Pastakhov:
add transclude function (v 3.5.0) requere PhpTags >= 5.1.0

https://gerrit.wikimedia.org/r/212750