diff --git a/includes/common.inc b/includes/common.inc
index 8849ef8..534bf3b 100644
--- a/includes/common.inc
+++ b/includes/common.inc
@@ -4798,14 +4798,124 @@ function drupal_clear_js_cache() {
 /**
  * Converts a PHP variable into its JavaScript equivalent.
  *
- * We use HTML-safe strings, i.e. with <, > and & escaped.
+ * We use HTML-safe strings, with several characters escaped.
  *
  * @see drupal_json_decode()
+ * @see drupal_json_encode_helper()
  * @ingroup php_wrappers
  */
 function drupal_json_encode($var) {
-  // json_encode() does not escape <, > and &, so we do it with str_replace().
-  return str_replace(array('<', '>', '&'), array('\u003c', '\u003e', '\u0026'), json_encode($var));
+  // PHP version cannot change within a request.
+  static $php530;
+
+  if (!isset($php530)) {
+    $php530 = version_compare(PHP_VERSION, '5.3.0', '>=');
+  }
+
+  if ($php530) {
+    // Encode <, >, ', &, and " using the json_encode() options parameter.
+    return json_encode($var, JSON_HEX_TAG | JSON_HEX_APOS | JSON_HEX_AMP | JSON_HEX_QUOT);
+  }
+
+  // json_encode() escapes <, >, ', &, and " using its options parameter, but
+  // does not support this parameter prior to PHP 5.3.0.  Use a helper instead.
+  return drupal_json_encode_helper($var);
+}
+
+/**
+ * Helper for drupal_json_encode on PHP versions below 5.3.0.
+ *
+ * @see drupal_json_encode()
+ */
+function drupal_json_encode_helper($var) {
+  switch (gettype($var)) {
+    case 'boolean':
+      return $var ? 'true' : 'false'; // Lowercase necessary!
+
+    case 'integer':
+    case 'double':
+      return $var;
+
+    case 'resource':
+    case 'string':
+      // Always use Unicode escape sequences (\u0022) over JSON escape
+      // sequences (\") to prevent browsers interpreting these as
+      // special characters.
+      $replace_pairs = array(
+        // ", \ and U+0000 - U+001F must be escaped according to RFC 4627.
+        '\\' => '\u005C',
+        '"' => '\u0022',
+        "\x00" => '\u0000',
+        "\x01" => '\u0001',
+        "\x02" => '\u0002',
+        "\x03" => '\u0003',
+        "\x04" => '\u0004',
+        "\x05" => '\u0005',
+        "\x06" => '\u0006',
+        "\x07" => '\u0007',
+        "\x08" => '\u0008',
+        "\x09" => '\u0009',
+        "\x0a" => '\u000A',
+        "\x0b" => '\u000B',
+        "\x0c" => '\u000C',
+        "\x0d" => '\u000D',
+        "\x0e" => '\u000E',
+        "\x0f" => '\u000F',
+        "\x10" => '\u0010',
+        "\x11" => '\u0011',
+        "\x12" => '\u0012',
+        "\x13" => '\u0013',
+        "\x14" => '\u0014',
+        "\x15" => '\u0015',
+        "\x16" => '\u0016',
+        "\x17" => '\u0017',
+        "\x18" => '\u0018',
+        "\x19" => '\u0019',
+        "\x1a" => '\u001A',
+        "\x1b" => '\u001B',
+        "\x1c" => '\u001C',
+        "\x1d" => '\u001D',
+        "\x1e" => '\u001E',
+        "\x1f" => '\u001F',
+        // Prevent browsers from interpreting these as as special.
+        "'" => '\u0027',
+        '<' => '\u003C',
+        '>' => '\u003E',
+        '&' => '\u0026',
+        // Prevent browsers from interpreting the solidus as special and
+        // non-compliant JSON parsers from interpreting // as a comment.
+        '/' => '\u002F',
+        // While these are allowed unescaped according to ECMA-262, section
+        // 15.12.2, they cause problems in some JSON parsers.
+        "\xe2\x80\xa8" => '\u2028', // U+2028, Line Separator.
+        "\xe2\x80\xa9" => '\u2029', // U+2029, Paragraph Separator.
+      );
+
+      return '"' . strtr($var, $replace_pairs) . '"';
+
+    case 'array':
+      // Arrays in JSON can't be associative. If the array is empty or if it
+      // has sequential whole number keys starting with 0, it's not associative
+      // so we can go ahead and convert it as an array.
+      if (empty($var) || array_keys($var) === range(0, sizeof($var) - 1)) {
+        $output = array();
+        foreach ($var as $v) {
+          $output[] = drupal_json_encode_helper($v);
+        }
+        return '[ ' . implode(', ', $output) . ' ]';
+      }
+      // Otherwise, fall through to convert the array as an object.
+
+    case 'object':
+      $output = array();
+      foreach ($var as $k => $v) {
+        $output[] = drupal_json_encode_helper(strval($k)) . ':' . drupal_json_encode_helper($v);
+      }
+      return '{' . implode(', ', $output) . '}';
+
+    default:
+      return 'null';
+  }
 }
 
 /**
diff --git a/modules/simpletest/tests/common.test b/modules/simpletest/tests/common.test
index 177e457..ccd3e34 100644
--- a/modules/simpletest/tests/common.test
+++ b/modules/simpletest/tests/common.test
@@ -2317,8 +2317,10 @@ class DrupalJSONTest extends DrupalUnitTestCase {
       $str .= chr($i);
     }
     // Characters that must be escaped.
-    $html_unsafe = array('<', '>', '&');
-    $html_unsafe_escaped = array('\u003c', '\u003e', '\u0026');
+    // We check for unescaped " separately.
+    $html_unsafe = array('<', '>', '\'', '&');
+    // The following are the encoded forms of: < > ' & "
+    $html_unsafe_escaped = array('\u003C', '\u003E', '\u0027', '\u0026', '\u0022');
 
     // Verify there aren't character encoding problems with the source string.
     $this->assertIdentical(strlen($str), 128, t('A string with the full ASCII table has the correct length.'));
@@ -2330,6 +2332,11 @@ class DrupalJSONTest extends DrupalUnitTestCase {
     $json = drupal_json_encode($str);
     $this->assertTrue(strlen($json) > strlen($str), t('A JSON encoded string is larger than the source string.'));
 
+    // The first and last characters should be ", and no others.
+    $this->assertTrue($json[0] == '"', t('A JSON encoded string begins with ".'));
+    $this->assertTrue($json[strlen($json) - 1] == '"', t('A JSON encoded string ends with ".'));
+    $this->assertTrue(substr_count($json, '"') == 2, t('A JSON encoded string contains exactly two ".'));
+
     // Verify that encoding/decoding is reversible.
     $json_decoded = drupal_json_decode($json);
     $this->assertIdentical($str, $json_decoded, t('Encoding a string to JSON and decoding back results in the original string.'));
diff --git a/modules/taxonomy/taxonomy.test b/modules/taxonomy/taxonomy.test
index aa7cc2e..597bfc3 100644
--- a/modules/taxonomy/taxonomy.test
+++ b/modules/taxonomy/taxonomy.test
@@ -627,11 +627,11 @@ class TaxonomyTermTestCase extends TaxonomyWebTestCase {
     }
     $this->assertNoText($term1->name, t('The deleted term %name does not appear on the node page.', array('%name' => $term1->name)));
 
-    // Test autocomplete on term 2 - it contains a comma, so expect the key to
-    // be quoted.
+    // Test autocomplete on term 2, which contains a comma.
+    // The term will be quoted, and the " will be encoded in unicode (\u0022).
     $input = substr($term2->name, 0, 3);
     $this->drupalGet('taxonomy/autocomplete/taxonomy_' . $this->vocabulary->machine_name . '/' . $input);
-    $this->assertRaw('{"\"' . $term2->name . '\"":"' . $term2->name . '"}', t('Autocomplete returns term %term_name after typing the first 3 letters.', array('%term_name' => $term2->name)));
+    $this->assertRaw('{"\u0022' . $term2->name . '\u0022":"' . $term2->name . '"}', t('Autocomplete returns term %term_name after typing the first 3 letters.', array('%term_name' => $term2->name)));
 
     // Test autocomplete on term 3 - it is alphanumeric only, so no extra
     // quoting.