PHP Basics

Languages - PHP
Basics

PHP is a scripting language developed specifically to produce web pages - the expected output is html. As a result, many php files contain both program code and html.

Generic info - Basics | Arrays | Functions | if statements | Objects

Code

PHP pages contain a mixture of HTML and PHP code. By default, everything in the file is HTML unless it is located between special tags.

All php commands are between <?php and ?>
All php commands must end with a semicolon
Code blocks (used with if and while statements) begin and end with curly braces

Everything is html until the following <?php echo "php code goes here"; ?> now more html <?php // this is a block of php code echo "New paragraph"; if ($a == 5){ echo "the condition was true"; echo "\n The if statement condition must be in parentheses"; echo "\n Curly braces define the begin and end of a block of code "; } ?>

Details of using the echo are covered at Debug.html.

Comments

PHP files may have both PHP and HTML comments.

PHP Comments	HTML Comments
// single line comment # a single line comment /* block comment */	<! simple comment - can not contain other tags > <!-- The only officially allowed comment Can contain <b>various tags</b> -->

Variables

All variables are variants and start with a dollar sign ($).

$a = "some string"; // basic assignment statement

echo a$; // include the string in the output

$a = 5;  // $a can be a string, number, array, anything

Constants are named with all uppercase letters and used without a dollar sign.

define ("DB_SERVER", "localhost");
define ("DB_USER",   "read_only");

$connection = mysql_connect(DB_SERVER,DB_USER,DB_PASS);

Functions

All variables used inside a function are local - by default. There are 2 exceptions

Those passed as function parameters
Those declared to be global

In the following example (from Debug.html - see that for an explanation), show the basic structure.

function mc_generic_function($input_string){ global $debug; global $debug_str; $a = "Start of mc_generic_function(\$input_string)\n"; $debug_str .= " $a"; // add entry to trace string if ($debug) {echo "$a";} // show it right now // the rest of the function goes here return = "some result"; // the return type is a variant - be careful }

To modify one of the passed parameters, it must be passed by reference (place an ampersand in front of the name).

function mc_generic_function(&$read_write_string){ $read_write_string = "some result"; // the new value will be available outside the function }

For additional examples, see Functions.

Strings

Either single quotes or double quotes can be used to define a string. When single quotes are used, the resultant string is exactly what is typed.

However, with double quotes, most (not all) variables present in the string will be evaluated and the result placed in the string.

The following example shows 3 ways to mix a variable with a string

The first one uses a period for concatenation
The second places the variable inside the double quotes
The third also adds curly braces around the variable

echo $variable . " more text"; // concatenation - via a period

echo "$variable more text";    // concatenation - via variable substitution

echo "{$variable} more text";  // concatenation, makes the variable clear
                               // useful if there is no trailing space
                               // variable substitution does not work with single quotes

Inline substitution requires double quotes. To stop variable substitution, either

Place a backslash in front of the dollar sign, or
Use single quotes

echo "\$variable more text";   // escaped $     -> $variable more text

echo '$variable more text';    // single quotes -> $variable more text

Some additional string functions

strlen($s)
trim($s)                remove white space

substr($s,$start,$len)  return substring
strpos
str_replace($a,$b,$s)   in $s, replace $a with $b

In query strings, back ticks are required for field names and many other places - be careful, even though they look similar, these are not the same as single quotes.

Include other files

There are 4 different methods to include one PHP file in another.

<?php require_once("includes/xyz.php"); ?>


include       Keep running if not found
require       Stop rendering the page if not found

include_once  Same as above, but only one time
require_once

In general, the library files which contain code should be required and included only once. As the example shows, these are usually placed in a subdirectory. I suggest having one directory for library files you use in multiple projects, and a separate directory for those that are specific to the current project.

try .. catch

Exceptions

    try{
      // try something here
    }catch (Exception $e){
      // this runs when there is a problem
    }

Built-in Exceptions Types

Author: Robert Clemenzi

Languages - PHP Basics