PHP: Testing and Debugging

Derek Bridge

Department of Computer Science,
University College Cork

PHP: Testing and Debugging

Aims:

• to learn how to test programs that use conditionals
• to learn how to debug programs that use conditionals

Class exercise

• Following the latest budget, the citizens of Nerdland pay 5% tax on the first 20,000 eurines that they earn; and they pay tax at 20% on the remainder
• Suppose salary_form.html contains this form:

  <form action="tax.php" method="get">
      <input type="text" name="salary" value="0" />
      <input type="submit" />
  </form>

• Using a two-armed conditional, complete tax.php (next slide), which outputs the amount of tax the user must pay at each rate

Incomplete version of tax.php

$salary = (int) $_GET['salary'];

$tax_paid_at_lower_rate = 0.0;
$tax_paid_at_higher_rate = 0.0;

if 

    



    
    


echo "<table>";
echo "<tr><th>Tax paid at lower rate</td><td>{$tax_paid_at_lower_rate}</td></tr>";
echo "<tr><td>Tax paid at higher rate</td><td>{$tax_paid_at_higher_rate}</td></tr>";
echo "</table>";

Testing

• Once all compile-time errors have been eliminated, we must thoroughly test a program to reveal all
  • run-time errors
  • logic errors
• Your approach must switch from constructive to destructive!
• Always assume that your program contains one more error!
• Hence, a successful test is one that finds an error!!!

Testing

• Test your program using input data that is
  • normal
  • boundary
  • exceptional
• Class exercise: Hence, what test inputs would you use for tax.php?
• Also make sure that every path through your program has been tested

Debugging

• To track down bugs, it can be useful to know
  • which statements get reached
  • what the variables contain at different points
• Special debugging software can be used for this but it is unnecessary for our simple programs
• We can use
  • echo statements to see what gets reached
  • var_dump to inspect the type and contents of variables

Class exercise

• The next slide shows another effort at tax.php by a programmer who has chosen to avoid two-armed conditionals
• The program is nearly correct but not quite: we will test and debug it but still avoid a two-armed conditional

Buggy version of tax.php

$salary = (int) $_GET['salary'];

$tax_paid_at_lower_rate = 0.0;
$tax_paid_at_higher_rate = 0.0;

// Determine amount of income over 20000 and tax it at the higher rate
if ( $salary > 20000 )
{
    $salary_taxable_at_higher_rate = $salary - 20000;
    $tax_paid_at_higher_rate = $salary_taxable_at_higher_rate * 0.2;
}

// Tax the rest at the lower rate
$tax_paid_at_lower_rate = $salary * 0.05;
    
echo "<table>";
echo "<tr><th>Tax paid at lower rate</td><td>{$tax_paid_at_lower_rate}</td></tr>";
echo "<tr><td>Tax paid at higher rate</td><td>{$tax_paid_at_higher_rate}</td></tr>";
echo "</table>";