Editor interprets some PHP array indices in strings as constants about language-php

[postcasio]

This is correct. They are constants. See the Complex (curly) syntax section on http://php.net/manual/en/language.types.string.php

// This is wrong for the same reason as $foo[bar] is wrong  outside a string.
// In other words, it will still work, but only because PHP first looks for a
// constant named foo; an error of level E_NOTICE (undefined constant) will be
// thrown.
echo "This is wrong: {$arr[foo][3]}"; 

You should quote your array indices always, even when using interpolation.