Intro to PHP
1 Introduction
PHP Hypertext Preprocessor is just a language like any other. Learning it is no different
than learning C, or Java, etc. This document will describe the basic PHP syntax.
2 Comments
PHP comments are basically exactly the same as C++ comments. You can use either:
// some comment here
Or
/* some comment here */
They work exactly like in C++; one is for single line comments and another for multi-line
comments.
3 Variables
PHP variables are sort of similar to Perl variables. They start with a $ and are virtually
type-free. You also don’t need to declare variables before using them. So you could for
example say:
$blah = 7;
Or
$blah = "Hello World";
And PHP would just figure out what value $blah has.
5 LOOPS
4 Conditional Statements
PHP has the usual conditional statements, like if, switch, etc. They have a pretty obvious
syntax, like:
if($a < 5){
// ... stuff goes here...
}
Or
switch($a){
case 1:
// do stuff
break;
case 2:
// do stuff
break;
default:
// do stuff
}
There are also one-liners
$a = $b < 0 ? -$b : $b;
5 Loops
There are the general purpose for and while loops, with obvious formats. Ie:
for($i=0;$i<100;$i++){
print $i."\n";
}
And:
$i=0;
while($i<100){
print $i."\n";
$i++;
}
6 Strings
Strings act just like you’d expect strings to act. In other words, when you do:
$blah = "Hello World";
You get a string. To concatenate strings you can use the “dot” operator:
$blah = "Hello"." "."World".$a." ".$blah;
7 Arrays
One of the most useful data structures is a plain array. Unfortunately the way PHP treats
arrays is a bit confusing—until you’re used to it.
7.1 Creating
You create arrays via the array() keyword. For example, to create an array of values, you
can do:
$arr = array("gah","beh","d0h","g0h",1,3.14);
You can also declare an empty array via:
$arr = array();
To access array elements, you can just do:
$a = $arr[1];
Arrays indexes start at 0 (more on this later).
7.2 Appending
You can just set values in the array by assigning them. For example:
$arr[100] = "blah";
It doesn’t matter if the array isn’t that big (it will grow).
Sometimes it is useful to “append” (or “push”) a value onto the end of the array. You
can do that by:
$arr[] = "glah";
7.3 Hashes 7 ARRAYS
7.3 Hashes
It turns out that in PHP, arrays and hashes (maps) are the same. So for example, you can
use integer indexes to access values, but you can also use string keys. For example:
$arr["name"] = "John Doe";
How does this work exactly? Well, arrays in PHP are just hashes. So when you use an
integer index, that number becomes the ‘key’.
To create a ‘hash’, you can also use the array keyword:
$arr = array("name" => "John Doe", "age" => 32);
7.4 Working with Arrays
There are basically three array functions in PHP, they are:
1. list()
2. each()
3. count()
7.4.1 list()
The list() function presents an “lvalue” of an array—meaning that you can assign things
to it. So for example, let’s say you have an array:
$arr = array("gah","beh","d0h","g0h",1,3.14);
Then you can assign it to values via:
list($a,$b,$c) = $arr;
The values $a, $b, $c will grab the first, second, and third values, respectively. This comes
in handy with the use of each().
7.4.2 each()
The function each() returns key => value pairs. So for example, even any array, we can
print out its contents via:
while(list($key,$value) = each($arr)){
print("$key => $value\n");
}
There’s also the reset() function that resets the internal counter used by each(). So
for example, if you wanted to start to loop through the list once again, you should call
reset($arr), which would put you right at the beginning of the array.
8 FUNCTIONS
7.4.3 count()
The count() does the obvious thing: it counts the number of elements in the array. This may
seem trivial, but you must remember that we are talking about associative arrays—where
indexes may not be numeric.
For example, imagine you have an array like:
$arr = array(0=>"gah",2=>"beh",4=>"d0h",100=>"g0h");
If you run count($arr), it will return a 4. However, you cannot just loop through the array
using a for loop like:
for($i=0;$i<count($arr);$i++){
print($arr[$i]."\n");
}
The above will not work because $i will get values: 0, 1, 2, and 3, which aren’t the proper
indexes for that array.
8 Functions
PHP has functions, which are basically just chunks of code that you can execute whenever
you want. The general format is something like this:
function blah(){
// do stuff here.
}
Functions can also take parameters and return values, i.e.:
function blah($a,$b,$c){
// do stuff here, using $a,$b,$c.
return $glah;
}
One of the more useful features is defaults. You can specify a default value for some
parameter if nothing is specified. For example:
function blah($a = 0.5,$b = 100,$c = 5){
// do stuff here, using $a,$b,$c.
return $glah;
}
Now if someone just calls blah(), the parameter values will be set to those defaults.
8.1 Call by Reference 8 FUNCTIONS
8.1 Call by Reference
You can also pass parameters by reference as opposed to by value. You just add an & in
front of their name. For example:
function blah(&$a,$b,$c){
$a = $b + $c;
}
The above will change the value of $a, as far as the caller is concerned.
8.2 global
Whenever you create variables in a function, they’re local to that function. So for example,
if you have code like the following:
$a = 3.14;
function blah(){
print $a."\n";
}
The value of $a in the function will be undefined. It’s a new variable (as far as the function
is concerned). If you want to access the ‘global’ variable, you need to specify that before you
use it in the function. I.e:
$a = 3.14;
function blah(){
global $a;
print $a."\n";
}
This comes in handy in PHP, since your script gets a lot of parameters from the user via the
global variables, such as $_GET, $_POST, $_REQUEST, $_SESSION, etc.
8.3 Variable Arguments
Your functions can take any number of parameters. You can use the:
$arr = func_get_args();
To retrieve an array of arguments your function got. For example,
function blah(){
$arr = func_get_args();
for($i=0;$i<count($arr);$i++){
print($i.": ".$arr[$i]."\n");
}
}
If you call it via: blah(1,2,"g0h"), it will display all the arguments (and if you call it with
a different number of arguments, it will display all of those).
9 Objects
PHP has objects, which are basically enclosures or structures, that can have “methods”. To
define a class, you just do something like the following:
class User {
var $id;
var $username;
var $password;
var $name;
var $email;
}
You also generally include some functions, like a constructor, or some other functions:
class User {
var $id;
var $username;
var $password;
var $name;
var $email;
function User ($arr){
$this->id=$arr[’id’];
$this->username=$arr[’username’];
$this->password=$arr[’password’];
$this->name=$arr[’name’];
$this->email=$arr[’email’];
}
function getEmailLink(){
return "<a href=’mailto:".$this->email."’>".$this->name."</a>";
}
}
The constructor has the same name as the class, and obviously you can have other functions,
etc.
You create ‘new’ objects via the ‘new’ operator:
$user = new User();
9.1 Inheritance 11 INCLUDING FILES
You can also pass an array to set the class attributes to some particular value. You can
always access the values directly, via:
print $user->name;
Or call a function via:
echo( $user->getEmailLink() );
9.1 Inheritance
You can extend classes to accomplish “inheritance”.
class SomeUser extends User {
function getEmail(){
return $this->email;
}
}
Now you have a new class, SomeUser, that has all the methods that User has, as well as an
additional method.
10 Meta Information
PHP lets you find information about the program itself. There are many useful functions
that you should read up on:
• get_class($object): returns the name of the class of the object as string.
• get_parent_class($object): returns the name of parent class.
• method_exists($object,$method): returns true/false if a method exists.
• class_exists($class): returns true/false if a class exists.
• is_subclass_of($object,$class): returns true/false if object is subclass of class.
11 Including Files
You are encouraged to modularize your PHP code into separate files. You can include files
in other files via the include command. Ie:
include("header.php");
12 PHP Superglobals
This section is taken right out of the PHP documentation.
• $GLOBALS: Contains a reference to every variable which is currently available within the
global scope of the script. The keys of this array are the names of the global variables.
• $_SERVER: Variables set by the web server or otherwise directly related to the execution
environment of the current script.
• $_GET: Variables provided to the script via URL query string.
• $_POST: Variables provided to the script via HTTP POST.
• $_COOKIE: Variables provided to the script via HTTP cookies.
• $_FILES: Variables provided to the script via HTTP post file uploads.
• $_ENV: Variables provided to the script via the environment.
• $_REQUEST: Variables provided to the script via the GET, POST, and COOKIE input
mechanisms, and which therefore cannot be trusted.
• $_SESSION: Variables which are currently registered to a script’s session.
13 Conclusion
Well, that’s about all there is to PHP, language wise.
