Introduction to PHP (Part 2)

Supplementary Material

Provided for Reference. This material is no longer taught on this module.

Lecturer: Dr Chris P. Jobling.

More on the language features of PHP

We continue our review of the Basics of PHP with a discussion of arrays, functions and regular expressions.

Based on Chapter 12 of Robert W. Sebasta, Programming the World-Wide Web, 3rd Edition, Addison Wesley, 2006. and Chapter 12 of Chris Bates, Web Programming: Building Internet Applications, 3rd Edition, John Wiley, 2006.

Further features of the PHP language with examples

• Arrays
• Functions
• Pattern Matching

The Examples for PHP Part 2.

At the end of this lecture you should be able to answer these questions:

1. In what two ways can arrays in PHP be created?
2. What keys are used when an array is created but no keys are specified?
3. Must all of the values of an array be of the same type?
4. Must all of the keys of an array be of the same type?
5. What exactly do the array_keys and array_values functions do?

At the end of this lecture you should be able to answer these questions:

1. What exactly does the in_array function do?
2. Explain the actions of the implode and explode functions.
3. Describe the actions of the next, reset, and prev functions.
4. What are the syntax and semantics of the two forms of the foreach statement?
5. Describe the result of using the sort function on an array that has both string and numeric values.

At the end of this lecture you should be able to answer these questions:

1. What is the difference between the sort and asort functions?
2. What happens if a script defines the same function more than once?
3. Are function names case sensitive?
4. What value is returned by a function if its execution does not end by executing a return statement?

At the end of this lecture you should be able to answer these questions:

1. What are the two ways you can specify that a parameter is to be passed by reference?
2. How can a variable used outside a function be accessed by the function?
3. How can you define a variable in a function so that its lifetime extends beyond the time the function is in its first execution?

• Not like the arrays of any other programming language
• A PHP array is a generalization of the arrays of other languages
• A PHP array is really a mapping of keys to values, where the keys can be numbers (to get a traditional array) or strings (to get a associative array)

• Use the array() construct, which takes one or more key ⇒ value pairs as parameters and returns an array of them
• The keys are non-negative integer literals or string literals
• The values can be anything
• Example:

    $list = array(0 => "apples", 1 => "oranges", 2 => "grapes")

• This is a “regular” array of strings

• If a key is omitted and there have been integer keys, the default key will be the largest current key + 1
• If a key is omitted and there have been no integer keys, 0 is the default key
• If a key appears that has already appeared, the new value will overwrite the old one

• Arrays can have mixed kinds of elements
• Some examples:

    $list = array("make" => "Cessna", "model" => "C210", 
                  "year" => 1960, 3 => "sold");
 
    $list = array(1, 3, 5, 7, 9);
 
    $list = array(5, 3 => 7, 5 => 10, "month" => "May");
 
    $colours = array('red', 'blue', 'green', 'yellow');

• Use brackets:

    $list[4] = 7;
    $list["day"] = "Tuesday";
    $list[] = 17;

• If an element with the specified key does not exist, it is created
• If the array does not exist, the array is created

• The keys or values can be extracted from an array:

    $highs = array("Mon" => 74, "Tue" => 70,
                   "Wed" => 67, "Thu" => 62,
                   "Fri" => 65);
    $days = array_keys($highs);
    $temps = array_values($highs);

• An array can be deleted with unset:

    unset($list);
    unset($list[4]);  # No index 4 element now

• is_array($list) returns true if $list is an array
• in_array(17, $list) returns true if 17 is an element of $list

• explode(“ ”, $str) creates an array with the values of the words from $str, split on a space
• implode(“ ”, $list) creates a string of the elements from $list, separated by a space

• Use current and next:

    $colours = array("blue", "red", "green", "yellow");
    $colour = current($colours);
    print("$colour <br />");
    while ( $colour = next($colours) ) {
      print ("$colour <br />");
    }

• This does not always work – for example, if the value in the array happens to be FALSE

• Alternative: use each, instead of next:

    while ( $element = each($colours) ) {
      print ("$element['value'] <br />");
    }

• The prev function moves current backwards

• To implement stacks with an array use array_push($list, $element) and array_pop($list)

• To visit every element of an array use foreach (array_name as scalar_name) { … }:

    foreach ($colours as $colour) {
      print "Is $colour your favourite colour?<br />";
    }

• produces:

Is red your favourite colour?

Is blue your favourite colour?
Is green your favourite colour?
Is yellow your favourite colour?

• foreach can iterate through both keys and values:

    foreach ($colours as $key => $colour) { ... }

• Inside the compound statement, both $key and $colour are defined
• Example:

    $ages = array("Bob" => 42, "Mary" => 43);
    foreach ($ages as $name => $age) {
      print("$name is $age years old <br />");
    }

• sort sorts the values of an array, leaving the keys in their present order – intended for traditional arrays
  • e.g., sort($list);
• The sort function does not return anything
• Works for both strings and numbers, even mixed strings and numbers:

    $list = ('h', 100, 'c', 20, 'a');
    sort($list);
    // Produces ('a', 'c', 'h'‘, 20, 100)

• In PHP 4+, the sort function can take a second parameter, which specifies a particular kind of sort:

    sort($list, SORT_NUMERIC);

• asort sorts the values of an array, while keeping the key/value relationships – intended for associative arrays
• rsort sorts the values of an array into reverse order
• ksort sorts the elements of an array by the keys, maintaining the key/value relationships e.g.:

    $list("Fred" => 17, "Mary" => 21, "Bob" => 49, "Jill" => 28);
    ksort($list);
    // $list is now ("Bob" => 49, "Fred" => 17, "Jill" => 28, "Mary" => 21)

• krsort sorts the elements of an array by the keys into reverse order

• An example to illustrate several of the sorting functions: sorting.php (sorting.php @ localhost)

• Code:

<<!DOCTYPE html>
 
<!-- sorting.php - An example to illustrate several of the
     sorting functions -->
<html lang="en">
  <head> 
    <meta charset="utf-8" />
    <title> sorting.php - An example to illustrate several of the
     sorting functions </title>
  </head>
  <body>
    <?php
       $original = array("Fred" => 31, "Al" => 27,
                         "Gandalf" => "wizzard",
                         "Betty" => 42, "Frodo" => "hobbit");
    ?>
    <h4> Original Array </h4>
    <?php
      foreach ($original as $key => $value) {
         print("[$key] => $value <br />");
      }
    ?>
    <h4> Array sorted with sort </h4>
    <?php
      $new = $original;
      sort($new);
       foreach ($new as $key => $value) {
         print("[$key] = $value <br />");
       }
    ?>
    <h4> Array sorted with asort </h4>
    <?php
       $new = $original;
       asort($new);
       foreach ($new as $key => $value) {
         print("[$key] = $value <br />");
       }
    ?>
    <h4> Array sorted with ksort </h4>
    <?php
       $new = $original;
       ksort($new);
       foreach ($new as $key => $value) {
         print("[$key] = $value <br />");
       }
    ?>
  </body>
</html> 

• Result:

• Syntactic form:

    function function_name( [formal_parameters] ) {
      ...
    }

• Functions need not be defined before they are called (in PHP 3, they must)
• Function overloading is not supported¹⁾
• If you try to redefine a function, it is an error
• Functions can have a variable number of parameters
• Default parameter values are supported
• Function definitions can be nested
• Function names are not case sensitive
• The return function is used to return a value; if there is no return, there is no returned value

• If the caller sends too many actual parameters, the subprogram ignores the extra ones
• If the caller does not send enough parameters, the unmatched formal parameters are unbound
• The default parameter passing method is pass by value (one-way communication)

• To specify pass-by-reference, prepend an ampersand to the formal parameter:

    function addOne(&$param) {
       $param++;
    }
 
    $it = 16;
    addOne($it);  // $it is now 17

• If the function does not specify its parameter to be pass by reference, you can prepend an ampersand to the actual parameter and still get pass-by-reference semantics:

    function subOne($param) {
      $param--;
    }
 
    $it = 16;
    subOne(&$it);   // $it is now 15

• Any type may be returned, including objects and arrays, using the return
• If a function returns a reference, the name of the function must have a prepended ampersand:

   function &newArray($x) { ... }

• An undeclared variable in a function has the scope of the function
• To access a nonlocal variable, it must be declared to be global, as in:

   global $sum;

• Normally, the lifetime of a variable in a function is from its first appearance to the end of the function's execution:

    static $sum = 0;  # $sum is static

• PHP has two kinds:
  • POSIX
  • Perl-compatible (preg)

        preg_match(regex, str [,array])

• The optional array is where to put the matches

The word_table.php (word_table.php @ localhost) example illustrates the use of functions and regular expressions

• Code:

<!DOCTYPE html>
<!--    word_table.php
 
Uses a function to split a given string of text into
its constituent words. It also determines the frequency of
occurrence of each word. The words are separated by
whitespace or punctuation, possibly followed by whitespace.
The punctuation can be a period, a comma, a semicolon, a
colon, an exclamation point, or a question mark.
The main driver program calls the function and displays
the results.
-->
<html lang="en">
  <head>
    <meta charset="utf-8" />
    <title> Word Table </title>
  </head>
  <body>
    <?php
 
    // Function splitter
    //  Parameter: a string of text containing words and punctuation
    //  Returns: an array in which the unique words of the string are
    //           the keys and their frequencies are the values.
 
    function splitter($str) {
 
      // Create the empty word frequency array
 
      $freq = array();
 
      // Split the parameter string into words
 
      $words = preg_split("/[ .,;:!?]\s*/", $str);
 
      // Loop to count the words (either increment or initialize to 1)
 
      foreach ($words as $word) {
        $keys = array_keys($freq);
        if (in_array($word, $keys))
          $freq[$word]++;
        else
          $freq[$word] = 1;
      }
 
      return $freq;
    }#** End of splitter

    // Main test driver
 
    $str = "apples are good for you, or don't you like apples?
or maybe you like oranges better than apples";
 
    // Call splitter
 
    $tbl = splitter($str);
 
    // Display the words and their frequencies
 
    print "<br /> Word Frequency <br /><br />";
    $sorted_keys = array_keys($tbl);
    sort($sorted_keys);
    foreach ($sorted_keys as $word) {
      print "$word $tbl[$word] <br />";
    }
  ?>
  </body>
</html>

• Output of the example script

Word Frequency

apples 3
are 1
better 1
don't 1
for 1
good 1
like 2
maybe 1
or 2
oranges 1
than 1
you 3

Further features of the PHP language with examples

• Arrays
• Functions
• Pattern Matching

The Examples for PHP Part 2.

At the end of this lecture you should be able to answer these questions:

1. In what two ways can arrays in PHP be created?
2. What keys are used when an array is created but no keys are specified?
3. Must all of the values of an array be of the same type?
4. Must all of the keys of an array be of the same type?
5. What exactly do the array_keys and array_values functions do?

At the end of this lecture you should be able to answer these questions:

1. What exactly does the in_array function do?
2. Explain the actions of the implode and explode functions.
3. Describe the actions of the next, reset, and prev functions.
4. What are the syntax and semantics of the two forms of the foreach statement?
5. Describe the result of using the sort function on an array that has both string and numeric values.

At the end of this lecture you should be able to answer these questions:

1. What is the difference between the sort and asort functions?
2. What happens if a script defines the same function more than once?
3. Are function names case sensitive?
4. What value is returned by a function if its execution does not end by executing a return statement?

At the end of this lecture you should be able to answer these questions:

1. What are the two ways you can specify that a parameter is to be passed by reference?
2. How can a variable used outside a function be accessed by the function?
3. How can you define a variable in a function so that its lifetime extends beyond the time the function is in its first execution?

Write the following PHP functions and code to test them

1. Parameter: An array of strings. Return value: A list of the unique strings in the parameter array.
2. Parameter: an array of numbers. Return value: The average and median of the parameter array.
3. Parameter: A string of numbers separated by spaces. Return value: The first four-digit number in the string.

1. Modify the sample script word_table.php discussed in this lecture to place the output table in an XHTML table.

These and other PHP exercises are also available on the homework page.

PHP for Web Applications

• Queries and Query Strings
• Form Handling
• Files
• Storing Information about Client State
• Session Tracking

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