feature symfony#7194 Added docs for Yaml::PARSE_CONSTANT (javiereguiluz)

This PR was squashed before being merged into the 3.2 branch (closes symfony#7194).

Discussion
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Added docs for Yaml::PARSE_CONSTANT

This fixes symfony#7178.

Commits
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fe7f5c5 Added docs for Yaml::PARSE_CONSTANT

Author: xabbuh

components/yaml.rst

@@ -293,6 +293,17 @@ flag::
     //       Line
     //       String
 
+Parsing PHP Constants
+~~~~~~~~~~~~~~~~~~~~~
+
+By default, the YAML parser treats the PHP constants included in the contents as
+regular strings. Use the ``PARSE_CONSTANT`` flag and the special ``!php/const:``
+syntax to parse them as proper PHP constants::
+
+    $yaml = '{ foo: PHP_INT_SIZE, bar: !php/const:PHP_INT_SIZE }';
+    $parameters = Yaml::parse($yaml, Yaml::PARSE_CONSTANT);
+    // $parameters = array('foo' => 'PHP_INT_SIZE', 'bar' => 8);
+
 Syntax Validation
 ~~~~~~~~~~~~~~~~~