PHP Debugging Techniques

Programming Requirements:

End every statement with semicolon (though, not language constructs like loops and conditionals)
Balance quotation marks, parentheses, curly braces, and square brackets
Single quotation marks must be closed with single quotations marks, same with double quotation marks
Escape (\) single- and double-quoatation marks w/in strings

Debugging Techniques:

Run PHP scripts through URL
Know version of PHP: PHP_VERSION, or phpinfo()
Turn on display_errors
Use error_reporting(E_ALL)
Check HTML source code
Use comments
Use print (echo) statements
Use single quotation marks to print variables literally
Use double quotation marks to print variable values
Use die() and/or exit() language constructs
Use following functions to print array values: print_r($myArray), var_export($myArray), var_dump($myArray)

Testing PHP Code (errors.php):

<?php
  error_reporting(E_ALL); 
  ini_set("display_errors", 1); 
  include("file_with_errors.php");
?>

**Common PHP Errors**
Blank Page	HTML issue, or PHP error (and display_errors or error_reporting turned off)
Parse Error	Missing semicolon (;), unbalanced curly braces, parentheses, or quotation marks, or use of an unescaped quotation mark in a string (\")
Empty variable value	Missing $, misspelled or incorrect cased variable name, or inappropriate variable scope (when using functions)
Undefined variable	Reference made to variable before it is given a value, or an empty variable value
Call to undefined function	Misspelled function name, PHP not configured to use function (e.g., MySQL function must be explicitly allowed in configuration file), or include file containing function definition not included
Cannot redeclare function	Two definitions of user-defined function exist, check included files
Headers already sent	White space exists in script before PHP tags, data has already been printed, or a file has been included.

Debugging Examples:

Example1 (incorrect function name):

$firstName="John";
$nameLength=str_len($firstName);
$myVar="Hi, my first name is " . $nameLength . " letters (actually bytes) long.";

Solution: should be strlen(), not str_len()

Hi, my first name is 4 letters (actually bytes) long.

Example2 (using echo statements):

$myNum=10;
if($myNum > 2)
{
  //some code
}
else
{
  //some other code
}

Solution: Use echo statements w/in code blocks

$myNum=10;
if($myNum > 2)
{
  echo "true path..."
  //some code
}
else
{
  echo "false path..."
  //some other code
}

Example3 (use print_r(), var_export(), or var_dump() functions):

$myArray = array("Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun");

Solution: use print_r($myArray), var_export($myArray), or var_dump($myArray)

/*
print_r() output is more human-readable, and can store output into a variable, var_dump() cannot. 
var_dump displays data types, the others do not.
var_export() is similar to var_dump() with one exception: the returned representation is valid PHP code

NOTE:
var_export cannot handle recursive arrays.
Can't do var_export($_GLOBALS);, as $_GLOBALS contains a reference to itself
*/

print_r($myArray); 
var_export($myArray); 
var_dump($myArray);

Array ( [0] => Mon [1] => Tue [2] => Wed [3] => Thu [4] => Fri [5] => Sat [6] => Sun )

array ( 0 => 'Mon', 1 => 'Tue', 2 => 'Wed', 3 => 'Thu', 4 => 'Fri', 5 => 'Sat', 6 => 'Sun', )

array(7) { [0]=> string(3) "Mon" [1]=> string(3) "Tue" [2]=> string(3) "Wed" [3]=> string(3) "Thu" [4]=> string(3) "Fri" [5]=> string(3) "Sat" [6]=> string(3) "Sun" }

Example4 (use die() or exit() language constructs):
Stop processing code at some particular juncture.

Solution: use die() or exit() language constructs to stop processing

exit("stop here");

//or... 
//die("stop here");

stop here