Introduction

On Monday, we began introducing the PHP language.
We went over its history, where its used, and it variants.
We then talked about where to obtain it and common configuration settings.
We said programs are executed by processing a file containing php commands. Initially, files are processed in copy mode, which directly copies character by character the contents of the file to the output stream.
When the sequence <?php is seen interpreter mode is entered and php commands can be executed. PHP stays in that mode until a ?> is seen.
We said PHP has the primitive built-in types: Boolean, integer, double, and string as well as compound type array and object.
We said variables in PHP always begin with $ and otherwise have names like in Java. For example, $foo. We said assignment and operators are pretty much the same as in C/C++/Java.
We said '.' is used to concatenate string and we went over various ways to create strings in PHP and the different kinds of string literals: single quote - no variable interpolation, double quote which has variable interpolations, heredoc -- multiline with interpolation, nowdocs -- multiline without interpolation.
We begin today by looking at control structures in PHP.

Control Statements

PHP supports: if, elseif, else if, else: For example,

//if
if ($a) print "hello";
if ($a) {
   print "hello";
   print "there";
}
if ($a) {
   $b = 1;
   echo  $b;
} else if ($b) {
   $b = 2;
   echo $b;
} else {
   $c = 1;
   echo $c;
}
//switch case:
switch($a)
{
   case 5:
      echo "hello";
      /* if $a matches the label (5 in this case)
         then execute the code after it;
       */
      break; //if hit then jump out of switch
   case "a label made from a string":
      echo "weird labels still work";
      break;
   default:
      echo "if all else fails do this";
}

for:

for($a = 0; $a<10; $a++){echo "hello $a";}

while:
```
while(!$var) { /*do something*/}
```
do while:
```
do {/*do something */} while (!$var);
```
To make it easier to switch back and forth between copy and interpretive mode PHP has an alternative format for control statements illustrated by:
```
<?php if ($the_world_is_round == true): ?>
The world is not flat.
<?php endif; ?>
```

Arrays

Arrays can be declared with the syntax:

$a = array("hi", 1, 2);
//for PHP >= 5.4 you can also use the syntax:
$a = ["hi", 1, 2]; //this is what modern PHP should use

You can nest arrays:

$b = ["hi", [1,2], 2];
$c = ["a" => "c"];

You can dereference arrays as in most languages:
```
echo $a[0]." ".$c['a'];
```

You can cycle over elements of an array using foreach:

foreach($arr as $var){echo $var;}

foreach($assoc_arr as $key => $value){echo "Key $key Value $value";}

To quickly get elements out of arrays you can also use the list syntax:

list($first, $second, ) = $arr; 
//$first will be $arr[0], $second will be $arr[1].

More on 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){...}

Default Argument Values, Name Function Parameters

PHP supports both creating functions with default argument values and calling functions using named parameters:

function myadd($a = 1, $b = 2, $c =3) {
  echo ($a+$b+$c) . "\n";
}
myadd(); //outputs 6
myadd(c: 4); // outputs 7 (PHP 8)

In-Class Exercise

Write a PHP function which echos the words in a string in reverse order, capitalizing the first letter of each word.

I.e.,

the quick brown fox

becomes

Fox Brown Quick The

Please post your solution to the Sep 21 In-Class Exercise Thread.

Arrays, Functions

Outline