add note about PHP 7.3 flexible heredoc/nowdoc

Author: methane

peps/pep-0822.rst

@@ -226,8 +226,12 @@ C# 11 also introduced a similar feature called `Raw String Literals <https://lea
 `Swift <https://docs.swift.org/swift-book/documentation/the-swift-programming-language/stringsandcharacters/#Multiline-String-Literals>`__
 also support triple-quoted string literals that automatically remove indentation.
 
+PHP 7.3 introduced `Flexible Heredoc and Nowdoc Syntaxes <https://wiki.php.net/rfc/flexible_heredoc_nowdoc_syntaxes>`__
+Although it uses closing marker (e.g. ``<<<END ... END``) instead of triple quote,
+it removes indent from text too.
+
 Java and Julia uses the least-indented line to determine the amount of indentation to be removed.
-Swift and C# uses the indentation of the closing triple quotes, similar to this PEP.
+Swift, C#, and PHP uses the indentation of the closing triple quotes or closing marker.
 
 This PEP chose the Swift and C# approach because it is more simple and easy to explain.