Position and Display Properties

Last week, we were talking about various properties of a tag that can be set using cascading style sheets. The last thing we covered was how to set the background-image of an element (I will tend to use element and tag interchangeably). Today, let's start by considering how to position elements on the screen using CSS...

The position property can have the value relative (says to place element relative to the last element) or absolute (says to place element at some offset from top left of display).

For example,

<style type="text/css"> 
   p.first {position: absolute; top:1in; left:2in;}
   p.second {position: relative; left:-1in;}
</style> 
<p class="first">displays one inch down 
two inches from left of top of screen</p>
<p class="second">displays 1inch to the left 
of where the first paragraph finished</p>

The display property can be one of block, inline, or none. block causes the box model to be used, inline makes the element data flow like text, and none does not render the element. You can also set the property "visibility: hidden" which renders the element in the background color
Other useful properties are width and height which set and control respectively the width and height of an element.

Keyboard Shortcuts

We are now done talking about CSS.
As a brief interlude before beginning our next major topic -- which is PHP-- I thought I would point out a useful way to make web pages more accessible.
This is to use the accesskey attribute of an element within the web page:
```
<input type="text" name="fname" accesskey="f" />
```
Now when ALT-f (maybe CTRL-f on a Mac) is typed the focus will go directly to this element.

Introducing PHP

PHP was developed by Rasmus Lerdorf around 1994-1995.
PHP originally stood for Personal Home Page, later PHP became an acronym for PHP: Hypertext Processor.
It is one of the most common server-side programming languages.
According to Tiobe, it is ranked sixth in overall popularity amongst all languages for any use.
The Wikipedia article on PHP says over 20 million domains are hosted on servers with PHP enabled.
The most recent version of PHP is PHP 7. The big advantage of PHP 7 over its predecessor is that its interpreter is almost twice as fast.
If you believe in language benchmarks... PHP tends to be faster than Python.
One oft-touted advantage of PHP is its shared nothing architecture: Each PHP request is distinct and does not directly share memory/data with other requests. Shared data is stored in filesystem/memory cache/database/etc.
This can make PHP easier to debug and easier to scale.

Overview

The web site for PHP is http://www.php.net/ and its manual is at http://php.net/manual/en/index.php.
Facebook's hhvm is an alternative implementation to PHP that runs at speeds comparable to PHP 7. It also interprets the Hack language -- a typed variant of PHP. hhvm is used by Facebook and Wikipedia.
The web server determines that a page is PHP usually by looking for one of the extensions .php, .php3, .phtml
When called from the web server on a PHP page, the PHP processor takes a PHP document and starts processing in one of two modes.
When it sees XHTML it simply copies it to the output. (Copy mode).
When it sees PHP code it executes the code and writes the result to the output.(Interpretive mode)
PHP is typically interpreted but it also can be precompiled.
The syntax of PHP is closely related to Perl and Javascript.

Configuring PHP

PHP should be installed and running on your machines as of HW1.
PHP can be run either using the Apache module, from the command line, or using the built-in web-server that comes with PHP (> 5.4.4).

From the command line:

php name_of_script.php #runs name_of_script.php
php -a # run in enter interactive mode. will get a prompt...
php > echo "hello world";
hello world
php > quit
cd ~/public_html 
php -S localhost:8000 #starts built-in web server on port 8000
    # using current dir as document root

You can fine-tune how PHP runs by editing the file php.ini
A ';' is used to indicate a comment in this file.
The most common thing for people to change is to have a line:
```
short_open_tag = On
```
so that you can escape to a PHP script using <? rather than <?php
Unfortunately, using the short open tag can mean your script is not as portable so for this class we will leave this off.
Xampp sets things this way by default so for today we will not worry about this. -- Many open source projects prefer long tags for compatibility.
Another common thing to change is the amount of error reporting PHP gives.
By default, it is assumed we are in a production environment so none is given.
For today, we want error-reporting. So find the line:
```
;error_reporting = E_ALL & ~E_NOTICE
```
and uncomment it, restart Apache

General Syntax

Recall a section of PHP code is delimited with <?php ... code here ...?>
The command include("filename.php"); can be used to include one document in another.
If the filename.php can't be found the script continues. To cause the script to fail in this case, you could use require("filename.php");
To prevent multiple inclusions of the same file you could use include_once, or require_once.
All variables in PHP begin with a $ sign. Names of variables are otherwise alphanumeric and underscore strings beginning with a dollar sign: $my_var, $fooFOO1999, etc. The first character after the $ is not allowed to be numeric.
PHP can use either Perl, C++, or C comments: #, //, /* ... */
PHPDoc is a common source code documentor for PHP.
The PHP Framework Interoperability Group publishes many useful standard for PHP code. Two useful one are: PSR-1: Basic Coding Standard and PSR-2: Coding Style Guide.

Quiz

Which of the following statements is true?

To specify two class selectors should apply to a tag, one space separates them as the value of the class attribute as in: <some-tag class="selector-1 selector-2">
Server Side Includes support for-loops.
In the box model, padding is the distance between the border of a box and the outer edge.

Primitives

PHP has four scalar types: Boolean, integer, double, and string.
There are also two compound types: array and object and two special types: resource and null.
PHP is dynamically typed: it has no type definitions (we will see we can type hint object later).
An unassigned variable is sometimes called an unbound variable and has value null. This can be coerced to one of the other types depending on the context.
To check if a variable is currently bound you can use the isset($variable_name) function. This returns true or false (one of the two Boolean values, case-insensitive).
empty($variable_name) can be used for isset($variable_name) && $variable_name != null.
You can also call error_reporting(-1), to set PHP's error reporting level to report all errors, warnings, notices, etc. so that it prints unbound variables.

Integer, String, and Boolean Type

Integers in PHP correspond to C long's (signed), so their size depends on the size of a long on a given machine.
PHP's double type corresponds to C's double and literals follow the format for C (or Perl or Javascript) floating point literals: I.e., .345, 3E-7, etc.
Characters in PHP are single bytes (No Unicode! -- we'll talk about how to fake later).
String literals are built up out of these characters.
PHP distinguishes between single quote and double quote literals to decide whether to do variable interpolation.
```
$bob = 6;
echo "Bob=$bob"; // outputs Bob=6 
echo 'Bob=$bob'; // outputs Bob=$bob
```
There are only two values for Boolean's: true or false. As for coercion, the empty string or a "0" or a "0.0" is interpreted as false. Otherwise, strings evaluate to true. Integers and double evaluate to true as long as they aren't zero.

Arithmetic Operators,etc

PHP supports the usual arithmetic operators: +, -, *, /, %, ++, --.
PHP also does type coercion.
Division should be assumed to output a double if any fractional value exists.
Some useful predefined functions are: floor, ceil, round, srand, rand, abs, min, and max.
String concatenation is done with a period: "string1". "string2" is "string1string2".
Some useful string operations are strlen, strcmp, strpos, substr, chop, trim, ltrim, strtolower, strtoupper.
Type conversion can be done using expressions like (int)$sum or intval($sum) or settype($sum, integer).
You can also check the type with is_int, is_integer, is_long, etc.
Assignment operators are like in C, Java, etc.

Output

You can output text from PHP interpretive mode into the page using either of the commands: print or echo For example,
```
<p> A paragraph. Let's hop into interpretive mode: <?php
print "hi there"; ?> back to copy mode </p>
```

In addition, to single and double quotes, you can use Heredocs and Nowdocs to output longer strings:

$a = "hello";
//Heredocs
$str = <<<EOD
This string that lives
on multiple lines uses heredocs
syntax. $a interpolation happens 
EOD;
echo $str;
//Nowdocs
$str2 = <<<'EOD2'
$a interpolation doesn't happens
EOD2;
echo $str2;

You can buffer copy mode data and get it as a string:

<?php
ob_start();
?>
<p>Hello World</p>
<?php
$out = ob_get_clean();
$out = strtolower($out);
echo $out;

PHP also has useful functions for printing out the values of variables and arrays: var_dump, print_r.

Finish CSS, PHP Data Types, and PHP Functions

Outline