Handling ErrorsDuring 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.
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
<l>
<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>
<l>
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
<l>
<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>
<l>
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?
|