PHP Debug Tips

Languages - PHP
Debug Tips

The simplest way to debug PHP scripts is to use something like Eclipse.

This covers other techniques for those without Eclipse.

echo - Display text and variables

The primary technique is to simply output some text that says "I am here" - it just indicates where the code is executing..

  echo 'some string';                        // I place these throughout the code

  echo "<br>start of subroutine_name ";      // I use these to see which routines are called

It is also useful to see what the values of some variables are. Because these are displayed in the web page, adding spaces or a little html will make the output easier to read.

  echo '<br> $variable1 - '.$variable1;  // single quotes stop the variables from being evaluated
  echo "<br> \$variable2 - $variable2";  // double quotes allow the variable to be evaluated
                                         // the backslash makes the magic character print normally (\$)

  echo "<br> array1['id'] = {$array1['id']}";   // curly braces are required for arrays

  echo '<!-- rlc note - '.$_GET['page'].' -->'; // visible in the html but not displayed
                                                // the dot ( . ) means concatenate

If you uses double quotes, then variables will be converted and linebreaks (\n) will work.

  echo "debug_message - $debug_message \n";   // places a line break in the output
                                              // however <br> displays a line break

In various routines, you would add code similar to the following

  global $debug_message;
  $debug_message .= "<br>$sql"; // concatenate new info to the existing string

and display the results via

  echo "debug_message - $debug_message";

For those messages that flash by too fast to read, try Ctrl-Shift-PrtSc (to capture a screen image) and then paste into Windows Paint (under Accessories/Paint.)

var_dump

Sometimes, particularly with complex data types, echo won't display anything. In these cases, try var_dump. Using the simplexml example, var_dump displayed

object(SimpleXMLElement)#301 (4) { ["title"]=> string(11) "Forty What?" ["from"]=> string(3) "Joe" ["to"]=> string(4) "Jane" ["body"]=> string(57) " I know that's the answer -- but what's the question? " }

However, since the actual formatting (in the html file) was

object(SimpleXMLElement)#301 (4) {
  ["title"]=>
  string(11) "Forty What?"
  ["from"]=>
  string(3) "Joe"
  ["to"]=>
  string(4) "Jane"
  ["body"]=>
  string(57) "
  I know that's the answer -- but what's the question?
 "
}

the data can be displayed on more than one line by encapsulating var_dump in <pre> tags.

$xml = simplexml_load_string($string); echo '<pre>'; var_dump($xml); echo '</pre>';

As mentioned above, in this case, the echo command did not print anything.

echo $xml; // this shows nothing

Program Trace

Rather than adding and removing lots of echo commands, I prefer to design functions with built-in, program controlled, debug support. This way, with a complex application, it is much quicker to locate problems.

debug_constants.php <?php // constants used to control debug output // 07-11-12 start of file // 0 means - do not print debug output $debug = 0; // print when subroutine starts $debug_query = 0; // print query strings $debug_trace = 0; // print $debug_str at end of output $debug_str = ""; // every subroutine adds to this if (isset($_GET['debug'])) { $debug = 1; $debug_query = $_GET['debug']; } if (isset($_GET['trace'])) { $debug_trace = 1; } ?>

As shown, the idea is to set the 3 debug parameters based on a command line (url) parameters.

http://someplace.com/page.php?debug
http://someplace.com/page.php?debug=1
http://someplace.com/page.php?debug&trace
http://someplace.com/page.php?trace

To display the trace, place the following in footer.php.

if ($debug_trace){echo "$debug_str";}

In a production environment, it is a good idea to disable these features. I suggest just using another debug_constants.php file where the variables are not changed.

Functions

This is one way to instrument functions and use the variables defined in the previous section.

function mc_generic_function($input_string){ global $debug; global $debug_str; $a = "Start of mc_generic_function(\$input_string)\n"; $debug_str .= " $a"; // add entry to trace string if ($debug) {echo "$a";} // show it right now // the rest of the function goes here }

Notes:

\n produces a newline in the html file which helps when reading it
\$ displays a dollar sign and keeps the value of the variable from being displayed
$string_variable .= "some string"; // adds the string to the existing value

Queries

Since many web sites have a database backend, a significant number of problems are related to building queries. I have a separate page that discusses queries and suggests how to instrument them using the ideas presented above. The $debug_query flag provides an additional level of control.

Author: Robert Clemenzi