This may be fixed, but working syntax is

echo "${array['1a']}";
or
echo "{$array['1a']}";

You should use {} for complex expression. We need improvement for string offset, though.

php > echo "${str{1}}";
PHP Parse error:  syntax error, unexpected '{' in php shell code on line 1

php > echo "{$str{1}}";
b

The change for this bug may also affect the way we declare variables starting with a number: https://3v4l.org/dVGQD

> the parser seems to stop at the first alpha character
Keys like "a1a" work. The parser is deciding whether the key is a string or number based on the first character: starts with a letter then it's a string, starts with a number then it's a number. So "1a" looks like a number at first, but then it hits the 'a' and gets confused.

Given the normal string/int type juggling with array keys, I suspect treating the key always as a string would work.

It's actually slightly different.  The scanner is regex based, and
if it is in ST_VAR_OFFSET state, it checks for a decimal number
(and matches 1), so it yields a numeric string.  Then it proceeds,
and matches a label (a), and yields it as string.  The parser
doesn't expect that, though.  This might be fixable, but I see no
point in doing that, especially since the complex interpolation
syntax already supports this, and the docs state[1]:

| For anything more complex, you should use the complex syntax.

We need to document, though, what is more complex, i.e. what
exactly is supported as array index for the simple syntax.

[1] <https://www.php.net/manual/en/language.types.string.php#language.types.string.parsing>