Outline

• Arrays
• Quiz
• Pattern Matching

PHP Arrays

• At the end of last week we were talking about array in PHP and had just had a simple in-class exercise experimenting with foreach statements to draw a table based on an array.
• We begin today by continuing to look at arrays.
• Arrays in PHP can in some sense always be viewed as associative.
• For example, the array $arr = [1,2,3]; can also be written in PHP as:

  $arr = [0=> 1, 1=>2, 2=>3];
  

• We can do things like:

  $arr = ["joe"=> 5, "mary" =>6];
  

• To get the keys and values we can use the functions:

  $keys = array_keys($arr) and $values = array_values($arr);
  

• Arrays can also be created by an assignment:

  $barr[1] = 5; // creates array $barr if doesn't exist
  

• If we did the assignment $barr[] = 6; then since the argument to [] wasn't specified PHP will assign $barr[2] = 6;

Yet More on PHP Arrays

• You can call the unset function on an element in an array to delete it from the array:

  $list=[2,4,6,8];
  unset($list[2]);
  // $list is now [0=>2,1=>4,3=>8]
  

• Some useful array functions:
  • count($arr) -- returns the number of elements in the array $arr
  • is_array($my_var) -- returns whether $my_var contains something of array type
  • in_array($elt, $arr) -- does $elt belong to the array $arr
  • implode, explode -- unsplit/split arrays into elements
  • sort($arr) -- sort an array by value (lots of variant functions in PHP ksort, asort, krsort, arsort, etc)
• To see how implode/explode work consider:

  $str="this is a string";
  $words = explode(" ", $str); /*make an array of tokens 
   of items between consecutive spaces.
   So words is the array ["this", "is", "a", "string"]. */
  $str2 = implode(" ", $words); //undoes the explode. 
  

Iterating Through Arrays

• The function current($arr) can be used to return a pointer to the current element in an array $arr. The next($arr) function can be used to advance this pointer and get its value:

  $cities = ["San Jose", "San Diego"];
  echo current($cities); // prints San Jose
  $another = next($cities); // $another is now San Diego;
  

• There are also the functions each, prev, end, and reset to facilitate moving through array.
• The function each is similar to next except after advancing the current pointer, it returns the old pointer as a two element array consisting of a key/value pair.

Functions

• The general format of a PHP function declaration is:

  function nameOfFunction([parameter_list]){ 
  /* function definition */}
  

• For example,

  function inc($i){return ++$i;} 
  

• A return value can be sent back using a return call as in many programming languages.
• You can modularize your code by putting several function definitions into a file and then use the include (preferably require_once) function to include them into any document that needs those functions.
• It is completely fine and sometimes useful to use copy mode in a function:

  function helloWorld()
  {
      ?>
      <h1>Hello World</h1>
      <?php
  }
  

• Parameters are passed by value. So the function call:

  $b = inc($a); // leaves the value of $a unchanged
  

• You can call by reference by using an ampersand (before php7):

  $b = inc(&$a); //here the value of $a is changed (one is added to it).
  

• You can also create functions with pass by reference parameters: function inc(&$i){...}

Variable Scope

• The default scope of a variable in PHP is only within the function that it is used. That is local scope:

  $bob = 5;
  function test()
  { 
     $bob = 6; 
     echo $bob; //echo's 6
  }
  test();
  echo $bob; //echo's 5
  

• In order to access global variables within a local function one would need to use the global declaration:

  $bob = 5;
  function test()
  {
     global $bob; # if did not do bob would be NULL 
  
     echo $bob; 
  }
  test();
  

  PHP also supports static local variables. These preserve states between function calls:

  function addone() {
      static $count =0; echo $count++;
  }
  

Quiz

Which of the following statements is true?

1. Contents of a file portion being executed in PHP Copy mode are copied to the output stream as is.
2. To cycle through the elements of an array in PHP you should use a for-in loop.
3. Variable interpolation can occur within single quoted string literals in PHP.

Pattern Matching

• PHP supports Perl-style regular expression and POSIX regular expressions. For example,

  $fruits = preg_split("/\s+/", "apples oranges   banana");
  //notice the extra spaces between oranges and banana.
  

  would split on one or more white space characters. So in this case, $fruits = ['apples', 'oranges', 'banana'].
• There are also functions

  int preg_match ( string $pattern , string $subject [, array &$matches 
      [, int $flags = 0 [, int $offset = 0 ]]] )
  
  int preg_match_all ( string $pattern , string $subject [, array &$matches 
      [, int $flags = PREG_PATTERN_ORDER [, int $offset = 0 ]]] )
  

  that let you find matches to regular expressions. In the above, [] arguments are optional. preg_match returns 1 (if pattern found), 0 otherwise. preg_match_all returns the number of matches or false if the pattern is not found. If a variable $matches is supplied to the function call, then it will be populated with either the first match found in preg_match case, or all matches in preg_match_all case. $flags controls the format of how $matches will be populated, and $offset says at what character to begin the search in $subject.
• There is a preg_replace($pattern, $replacement, $subject) function that let's you substitute regex $pattern matches with a string $replacement of your choice.