PHP BASICS

In this part of the PHP tutorial, we will talk about basic programming in PHP. 

All the PHP code is surrounded by two delimiters, <?php and ?>. The 'php' string is optional but recommended. 

<?php 

# PHP code

?>

PHP code is put between two delimiters. 

<?php 

$a = 23;
print $a;

?>

This PHP script assigns a value to a variable. It prints it to the console. Note that we say console, because here we use the PHP_CLI command line interpreter. If you test these examples on the web, the output will be sent to the browser. 

$a = 23;

We assign a value 23 to the $a variable. Each variable starts with a dollar character. This PHP code line is a statement. Each statement ends with a semicolon. In PHP, semicolons are not mandatory, like in Javascript or Ruby. They are obligatory. 

print $a;

We print the $a variable to the console. The print keyword does not add a new line to the output. If we want a new line, we must put it manually.print keyword takes only one argument. 

<?php 

$a = 23;
$b = 24;

echo $a, "\n", $b, "\n";

?>

In this script, we use the echo keyword. It is similar to the print keyword. Unlike the print keyword, it can take multiple arguments. 

$a = 23;
$b = 24;

We define two variables. 

echo $a, "\n", $b, "\n";

We print the variables to the console. We also include the new line characters. Arguments can be separated by commas. 

$ php echo.php 
23
24

This is the output of the script. 

TYPES

PHP is a weakly typed language. It works with types, but the programmer does not specify them when declaring variables. A data type is a one of various types of data, as double, integer, or boolean. Values of a certain data type are from a specific range of values stating the possible values for that type, the operations that can be done on that type, and the way the values of that type are stored. PHP works implicitly with data types. Programmers do not specify explicitly the data types. 

<?php 

$a = "Jane";
echo "$a \n";

$a = 12;
echo "$a \n";

$a = 56.4;
echo "$a \n";

$a = true;
echo "$a \n";

?>

In this PHP script, we have an $a variable. First, we assign it a string, then an integer, a double and finally a boolean value. If we assign a string to a variable the PHP automatically creates a string variable. 

$ php dynamic.php 
Jane 
12 
56.4 
1 

Running the script. 

<?php 

$temperature = 12.4;
$name = "Jane";
$age = 17;
$values = array(1, 2, 3, 4, 5, 6); 

class Being {};

$somebody = new Being();

echo gettype($temperature), "\n";
echo gettype($name), "\n";
echo gettype($age), "\n";
echo gettype($values), "\n";
echo gettype($somebody), "\n";

?>

In the above PHP script, we dynamically create five types. 

$values = array(1, 2, 3, 4, 5, 6); 

class Being {};

This is an array and a class. Both types will be covered later in more detail. 

echo gettype($temperature), "\n";

The gettype() function returns the type of the variable in question. 

$ php gettype.php 
double
string
integer
array
object

This script lists the basic types of the PHP language. 

CONSTANTS

In PHP, we can create constants. A constant is a name for a value that, unlike a variable, cannot be reassociated with a different value. We use the define() function to create constants in PHP. 

<?php 

define("BLUE", "0000FF");

echo BLUE, "\n";

echo defined("BLUE");
echo "\n";

?>

In this PHP script, we define a BLUE constant. 

define("BLUE", "0000FF");

Here we define the BLUE constant. It is a convention to write constants in uppercase letters. 

echo BLUE, "\n";

Here we use it. Note that constants are not preceded by ($) dollar character. 

echo defined("BLUE");

We have used another function, the defined() function. It checks, if a particular constant exists. Returns true, if it does. 

$ php constant.php 
0000FF
1

Running the example gives the above output. 
PHP also has some predefined constants. 

<?php 

echo TRUE;
echo "\n";
echo PHP_VERSION;
echo "\n";
echo PHP_OS;
echo "\n";
echo __LINE__;
echo "\n";
echo __FILE__;
echo "\n";
echo DIRECTORY_SEPARATOR;
echo "\n";
echo PHP_DATADIR;
echo "\n";

?>

Here we print some built-in PHP constants. For example, the PHP_OSconstant prints the OS version on which the PHP was built. 

$ php constants.php 
1
5.2.6-2ubuntu4.6
Linux
9
/home/vronskij/programming/php/basics/constants.php
/
${prefix}/share

On my system, I get this output. 

VARIABLE INTERPOLATION

Next, we will define interpolation. Variable interpolation is replacing variables with their values inside string literals. Another names for variable interpolation are: variable substitution and variable expansion. 

<?php 

$age = 17;

echo "Jane is $age years old\n";

?>

The $age variable is replaced with the value 17 in the string. 

$ php interpolation.php 
Jane is 17 years old

<?php 

$age = 17;

echo 'Jane is $age years old\n';

?>

However, this does not work, if we use the single quotes. In this case, no interpolation happens and no special characters are working. 

$ php interpolation2.php 
Jane is $age years old\n$ 

We see a verbatim output of the string. 

INCLUDING FILES

PHP code is split in multiple files for bigger programs. We use the includestatement to join various PHP files. 

<?php 

define("VERSION", 1.12);

function get_max($x, $y) {
    if ($x > $y) {
        return $x;
    } else {
        return $y;
    }
}

?>

Let's say, we have a common.php file, in which we define some constants and functions. 

<?php 

include "common.php";

echo "The version is " . VERSION . "\n";

$a = 5;
$b = 3;

echo get_max($a, $b), "\n";

?>

And we have another file, which wants to use the aforementioned definitions. 

include "common.php";

We simply include the definitions to our file with the include keyword. We must specify the exact path to the common.php file. In our simple case, both files are in the same directory. 
This chapter covered some basics of the PHP language.