PHP: Types

Derek Bridge

Department of Computer Science,
University College Cork

PHP: Types

Aims:

• to learn about data types in PHP
• to learn about integers and floating-point numbers
• to learn about variables and their types

Data types

• A PHP program can compute with 8 different types of data
  • string: used for strings of chars
  • integer: used for whole numbers
  • float: used for real numbers
  • boolean: used for true and false values
  • array: used for collections of data
  • object: used for instances of classes
  • resource: used for external resources (such as database connections)
  • NULL: the special value NULL
• A data type is defined by giving a set of values, and the operations we can use to create, access and modify the values

PHP's string data type

• Values
  • Zero, one or more chars within quotes
  • E.g. '', ' ', 'a', 'Hello world!', "Hello world!", '1109',...
• Operators
  • Concatenation (.) and some others

PHP's numeric data types

• PHP has two numeric data types
  • integer (used for whole numbers)
  • float (used for real numbers)
• Operators
  • Addition (+), subtraction (-), multiplication (*), division (/), modulus, i.e. remainder (%), and some others
• Class exercise: Evaluate (a) 27 % 10, (b) 2 % 3
• (Lots of little details suppressed here, e.g. what if you divide by zero)

PHP's integer data type

• Values
  • Whole numbers, e.g. 12, 256, 0, -113
  • Typical range: -2,147,483,648 to +2,147,483,647
• (Lots of little details suppressed here, e.g. what if you try to store something that's too big)

PHP's float data type

• Values
  • Numbers with decimal digits
  • E.g. 7.2, 0.005, -1.3
  • E.g. (scientific notation) 0.72E1, 5E-3, -130E-2
  • Typical approximate range: from -1.7E+308 to -1.7E-308 and from 1.7E-308 to 1.7E+308 with perhaps 14 digits of precision
  • Note: 8 is an int but 8.0 and 8E0 are floats
  • Class exercise Which is the odd one out:
    8.0, 8E0, 0.08E2, 80E-1, 0.80E2
• (Lots of little details suppressed here, e.g. what if you try to store something that's too big)

Variables and their types

• Variables have a type as well as a name
• In PHP, the type of a variable is determined by what value is assigned to it
• Class exercise: Give the types of each variable below:

  $a = 3;
  $b = 4.5;
  $c = "plip";
  $d = 3.0;
  $e = 6 + 7;
  $f = 6 - $a;
  $g = $e * $f;
  $h = 2 + 1.5;
  $i = $a + $b;
  $j = 6 / 2;
  $k = 6 / 4;
  $l = 6 / 2.0;
  $m = $c . ' plop';
  $n = "{$c} plop";
  $o = "three";
  $p = "3";
  $q = '3.0';

Example program

• Suppose college_form.html contains this form:

  <form action="college_ratio.php" method="get">
      <input type="text" name="num_of_students" value="0" />
      <input type="text" name="num_of_staff" value="0" />
      <input type="submit" />
  </form>

• (We won't use HTML5's <input type="number" /> because browser support is too limited at present)
• We will write college_ratio.php, which computes and echoes the number of students per staff member

Form data arrives as strings

• You might start your script as follows:

  $num_of_students = $_GET['num_of_students'];
  $num_of_staff = $_GET['num_of_staff'];

• But form data arrives as strings
  • suppose the user enters 100 (num_of_students) and 4 (num_of_staff)
  • $_GET['num_of_students'] contains '100', not 100
  • hence $num_of_students will also contain '100', and will be of type string
  • but the later computations will require numbers, not strings
  • at some point, we need to convert the strings to ints
• (Form data arrives as strings —even if you use HTML5's <input type="number" />)

Explicit type conversion

• Use (int) to request type conversion to integer from, e.g., string
• Use (float) to request type conversion to float from, e.g., string
• E.g.

  $num_of_students = (int) $_GET['num_of_students'];
  $num_of_staff = (int) $_GET['num_of_staff'];

• Class exercise: So now what type of variable is $num_of_students?

The finished program

$num_of_students = (int) $_GET['num_of_students'];
$num_of_staff = (int) $_GET['num_of_staff'];

$num_of_students_per_staff = $num_of_students / $num_of_staff;
   
echo '<p>';
echo $num_of_students_per_staff;
echo '</p>';

But that's not the end of the story

• This is still not satisfactory: we can't leave the program like this
• Class exercise: What more should we be doing?
• We'll do this in a few lecture's time

Class exercise

• A student has to travel a certain distance to get to college (e.g. 1.7km) and must make this journey a certain number of times in a year (e.g. 240).
• Suppose journey_form.html contains this form:

  <form action="total_journey.php" method="get">
      <input type="text" name="distance" value="0.0" />
      <input type="text" name="num_of_trips" value="0" />
      <input type="submit" />
  </form>

• Write total_journey.php which calculates and outputs the total distance (km) that the student travels

Variables and their types

• In some programming languages
  • the programmer declares the type of the variable
  • must then only store values of that type in the variable
  • gets a fatal error (at compile-time or run-time, depending on the programming language) if s/he tries to store a value of a different type
• E.g. Java

  int qty = 7;
  qty = "Hello, world!";

  The second statement gives rise to a fatal error message at compile-time

Variables and their types

• In PHP
  • the type of the variable is determined by what value is assigned to it
  • you are allowed to replace a variable's value with one of another type
• E.g. PHP

  $qty = 7;
  $qty = "Hello world!";

  The second statement gives rise to no error message nor warning message
• We will exploit this in a highly-constrained way in our PHP scripts
• But advice in general: use a variable for only one purpose in your program