XML and PHP [Electronic resources] نسخه متنی

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XML and PHP [Electronic resources] - نسخه متنی

Vikram Vaswani

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Handling Errors



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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