Handling Errors | XML and PHP

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During the initial stages of application development, it's possible to get by without using error handlers in your code; however, as an application is being packaged for release, graceful error handling and recovery becomes a must.

PHP allows developers to accomplish this error handling via its xml_get_error_code() function (which prints the error code returned by the parser when it hits a bump) and its xml_error_string() function, (which returns a short, human-readable error message corresponding to the error code). Table 2.2 is a list of error codes and their corresponding named constants, together with what they mean.

Table 2.2. SAX Parser Error Codes

Error Code	Error Constant	Meaning
1	`XML_ERROR_NO_MEMORY`	Parser out of memory
2	`XML_ERROR_SYNTAX`	Syntax error
3	`XML_ERROR_NO_ELEMENTS`	No element found
4	`XML_ERROR_INVALID_TOKEN`	Document not well- formed
5	`XML_ERROR_UNCLOSED_TOKEN`	Unclosed token
6	`XML_ERROR_PARTIAL_CHAR`	Unclosed token
7	`XML_ERROR_TAG_MISMATCH`	Mismatched tag
8	`XML_ERROR_DUPLICATE_ATTRIBUTE`	Duplicate attribute
9	`XML_ERROR_JUNK_AFTER_DOC_ELEMENT`	Junk after document element
10	`XML_ERROR_PARAM_ENTITY_REF`	Illegal parameter entity reference found
11	`XML_ERROR_UNDEFINED_ENTITY`	Undefined entity
12	`XML_ERROR_RECURSIVE_ENTITY_REF`	Recursive entity reference
13	`XML_ERROR_ASYNC_ENTITY`	Asynchronous entity
14	`XML_ERROR_BAD_CHAR_REF`	Reference to invalid character number found
15	`XML_ERROR_BINARY_ENTITY_REF`	Reference to binary entity found
16	`XML_ERROR_ATTRIBUTE_EXTERNAL_ ENTITY_REF`	Reference to external entity found within attribute
17	`XML_ERROR_MISPLACED_XML_PI`	XML processing instruction not found at start of external entity
18	`XML_ERROR_UNKNOWN_ENCODING`	Unknown document encoding
19	`XML_ERROR_INCORRECT_ENCODING`	Incorrect document encoding specified
20	`XML_ERROR_UNCLOSED_CDATA_SECTION`	CDATA section not closed correctly
21	`XML_ERROR_EXTERNAL_ENTITY_HANDLING`	Error in processing external entity reference

In order to illustrate how this error-handling works, consider Listing 2.19, which contains a badly formed XML document. (There's a duplicate attribute within the element <circle> .) Note that the xml_error_string() function has been used to return a more helpful description of the error.

Listing 2.19 Better Error Handling

 <html>  <head>  <basefont face="Arial">  </head>  <body>  <?php  // XML data  $xml_data = <<<EOF  <?xml version="1.0"?>  <shape>        <circle color="red" radius="5" x="14" y="574" color="red" />  </shape>  EOF;  // define handlers  function startElementHandler($parser, $name, $attributes)  {       // code snipped out  }  function endElementHandler($parser, $name)  {       // code snipped out  }  // initialize parser  $xml_parser = xml_parser_create();  // set callback functions  xml_set_element_handler($xml_parser, "startElementHandler", "endElementHandler");    if (!xml_parse($xml_parser, $xml_data))  {       $ec = xml_get_error_code($xml_parser);        die("XML parser error (error code $ec): " . xml_error_string($ec));  }  // all done, clean up!  xml_parser_free($xml_parser);  ?>  </body>  </html>

Why stop there? It's also possible to include information on where exactly the error occurred within the document, with these three built-in functions:

xml_get_current_line_number() ” Returns the current line number
xml_get_current_column_number() ” Returns the current column number
xml_get_current_byte_index() ” Returns the current byte offset

Listing 2.20 is the revised script that incorporates this information.

Listing 2.20 Adding Line and Column Information to Error Messages

 <html>  <head>  <basefont face="Arial">  </head>  <body>  <?php  // XML data  $xml_data = <<<EOF  <?xml version="1.0"?>  <shape>        <circle color="red" radius="5" x="14" y="574" color="red" />  </shape>  EOF;  // define handlers  function startElementHandler($parser, $name, $attributes)  {       // code snipped out  }  function endElementHandler($parser, $name)  {       // code snipped out  }  // initialize parser  $xml_parser = xml_parser_create();  // set callback functions  xml_set_element_handler($xml_parser, "startElementHandler", "endElementHandler");  if (!xml_parse($xml_parser, $xml_data))  {       $ec = xml_get_error_code($xml_parser);        die("XML parser error (error code $ec): " . xml_error_string($ec) . "<br>Error  occurred at line " . xml_get_current_line_number($xml_parser) . ", column " .  xml_get_current_column_number($xml_parser) . ", byte offset " .  xml_get_current_byte_index($xml_parser));  }  // all done, clean up!  xml_parser_free($xml_parser);  ?>  </body>  </html>

And here's the output:

 XML parser error (error code 8): duplicate attribute  Error occurred at line 3, column 46, byte offset 78

There! Isn't that much more helpful?

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