This is the first entry listed under the "Backward Incompatible Changes" section of the 7.3 migration guide.

http://www.php.net/manual/en/migration73.incompatible.php

Oh thank you so much. I didn't look there.
Instead, was staring at https://www.php.net/manual/en/language.types.string.php#language.types.string.syntax.heredoc which doesn't even mention new syntax.
Thanks again.

> […] which doesn't even mention new syntax.

Re-opening as doc bug, then.

The documentation at https://www.php.net/manual/en/migration73.incompatible.php#migration73.incompatible.core.heredoc-nowdoc is definitely wrong.

As written:
<?php
$str = <<<FOO
abcdefg
   FOO
FOO;
?>

will not produce the error mentioned. Instead, it will give the following:
Parse error: Invalid body indentation level (expecting an indentation level of at least 4)

If you don't indent that line:
<?php
$str = <<<FOO
abcdefg
FOO
FOO;
?>

You'll get an error, but still not the one reported. This appears to be a bug though, as it's requiring a semicolon after the identifier, despite the fact that documentation says that is no longer required. See https://bugs.php.net/bug.php?id=76608

So, it doesn't appear the other issue I mentioned in my previous email is actually a bug. The fact that the migration documentation is incorrect still applies, though.

The problem in php interactive mode is still there. Here I show How I create a php file and add a simple function that echoes a string from a heredoc.
But the same function in interactive mode is not even recognized as a complete heredoc, so I cannot even close the function. see this:

santiagoarizti@santis-mac:~$ docker run --rm -it php:7.3 bash
root@a8df21f382dd:/# echo -e '#!/usr/local/bin/php\n<?php\nfunction sayHello($name) {\n  echo <<<EOF\n    hola $name\n    EOF;\n}\nsayHello($argv[1]);echo PHP_EOL;\n' > santi.php
root@a8df21f382dd:/# cat santi.php 
#!/usr/local/bin/php
<?php
function sayHello($name) {
  echo <<<EOF
    hola $name
    EOF;
}
sayHello($argv[1]);echo PHP_EOL;

root@a8df21f382dd:/# php santi.php Santiago
hola Santiago
root@a8df21f382dd:/# php -a
Interactive shell

php > function sayHello($name) {
php {   echo <<<EOF
<<< >     hola $name
<<< >     EOF;
<<< > }

Notice how I am still inside heredoc according to php.

Here is a simpler example without anything but an echo:

santiagoarizti@santis-mac:~$ docker run --rm -it php:7.3     
Interactive shell

php > echo <<<EOF
<<< > this works fine, 
<<< > old syntax
<<< > EOF;
this works fine,
old syntax
php > echo <<<EOF
<<< >   this doesn't
<<< >   work, new syntax
<<< >   EOF;
<<< >

Ok, after a little bit more playing around with interactive mode, I found the problem, but I cannot fix it. The php interpreter is ok, it is just the interactive mode that is not recognizing the end of the flexible heredoc. But, with this combination of lines the interpreter behaves "correctly"

santiagoarizti@santis-mac:~$ docker run --rm -it php:7.3
Interactive shell

php > echo <<<EOF
<<< >   let us see
<<< >   EOF /*
<<< > EOF
php > */
php > ;
let us see
php > 

I gave the interpreter the old expected ending, but inside a multiline comment, so it doesn't complain about a missing constant... perhaps that gives an idea of what is causing the issue

Would it be worth splitting this into two issues? This one is currently classified as a documentation issue, but the interactive mode issue seems quite distinct.

I've split the doc bug out into bug #78425, since this report has a fair bit of commentary about the interactive mode bug.

Looks like the fix will be to ext/readline's cli_is_valid_code().

I've just had a look around and it might be a bit fiddly to fix. Around line 354 of ext/readline/readline_cli.c.
Allowing whitespace between the start of line and the heredoc tag shouldn't be too difficult, but the new syntax also allows the tag to be followed by code other than just `\n` or `;\n` - will need some way to distinguish `   END, $somethingElse);` from `    ENDING inside the heredoc`.