[Yaml] Improve YAML boolean escaping

- Moves dumping single-quoting logic into Yaml\Escaper
- Ensures that PHP values which would be interpreted as booleans in
  older versions of the YAML spec are escaped with single quotes when
  dumped by the Dumper.

src/Symfony/Component/Yaml/Escaper.php

@@ -72,7 +72,7 @@ class Escaper
      */
     public static function requiresSingleQuoting($value)
     {
-        return preg_match('/[ \s \' " \: \{ \} \[ \] , & \* \# \?] | \A[ \- ? | < > = ! % @ ` ]/x', $value);
+        return self::containsCharRequiresSingleQuoting($value) || self::isValueRequiresSingleQuoting($value);
     }
 
     /**
@@ -86,4 +86,28 @@ class Escaper
     {
         return sprintf("'%s'", str_replace('\'', '\'\'', $value));
     }
+
+    /**
+     * Determines if a PHP value contains any single characters that would cause
+     * the value to require single quoting in YAML.
+     *
+     * @param  string $value A PHP value
+     * @return bool          True if the value would require single quotes.
+     */
+    private static function containsCharRequiresSingleQuoting($value)
+    {
+        return preg_match('/[ \s \' " \: \{ \} \[ \] , & \* \# \?] | \A[ \- ? | < > = ! % @ ` ]/x', $value);
+    }
+
+    /**
+     * Determines if a PHP value is entirely composed of a value that would
+     * require require single quoting in YAML.
+     *
+     * @param  string $value A PHP value
+     * @return bool          True if the value would require single quotes.
+     */
+    private static function isValueRequiresSingleQuoting($value)
+    {
+        return in_array(strtolower($value), array('null', '~', 'true', 'false', 'y', 'n', 'yes', 'no', 'on', 'off'));
+    }
 }

src/Symfony/Component/Yaml/Inline.php

@@ -135,12 +135,10 @@ class Inline
             case Escaper::requiresDoubleQuoting($value):
                 return Escaper::escapeWithDoubleQuotes($value);
             case Escaper::requiresSingleQuoting($value):
+            case preg_match(self::getTimestampRegex(), $value):
                 return Escaper::escapeWithSingleQuotes($value);
             case '' == $value:
                 return "''";
-            case preg_match(self::getTimestampRegex(), $value):
-            case in_array(strtolower($value), array('null', '~', 'true', 'false')):
-                return "'$value'";
             default:
                 return $value;
         }

src/Symfony/Component/Yaml/Tests/InlineTest.php

@@ -190,6 +190,14 @@ class InlineTest extends \PHPUnit_Framework_TestCase
             "'#cfcfcf'" => '#cfcfcf',
             '::form_base.html.twig' => '::form_base.html.twig',
 
+            // Pre-YAML-1.2 booleans
+            "'y'" => 'y',
+            "'n'" => 'n',
+            "'yes'" => 'yes',
+            "'no'" => 'no',
+            "'on'" => 'on',
+            "'off'" => 'off',
+
             '2007-10-30' => mktime(0, 0, 0, 10, 30, 2007),
             '2007-10-30T02:59:43Z' => gmmktime(2, 59, 43, 10, 30, 2007),
             '2007-10-30 02:59:43 Z' => gmmktime(2, 59, 43, 10, 30, 2007),
@@ -257,6 +265,14 @@ class InlineTest extends \PHPUnit_Framework_TestCase
             "'-dash'" => '-dash',
             "'-'" => '-',
 
+            // Pre-YAML-1.2 booleans
+            "'y'" => 'y',
+            "'n'" => 'n',
+            "'yes'" => 'yes',
+            "'no'" => 'no',
+            "'on'" => 'on',
+            "'off'" => 'off',
+
             // sequences
             '[foo, bar, false, null, 12]' => array('foo', 'bar', false, null, 12),
             '[\'foo,bar\', \'foo bar\']' => array('foo,bar', 'foo bar'),