String Manipulation
http://www.phpnoise.com/tutorials/25/1

Introduction
The nature of web development requires strings to be created, manipulated and, in some cases, evaluated. Understanding the concept of string manipulation is an important piece for developers.
This introductory tutorial will explain some simple constructs associated to handling strings in addition to some more advanced techniques for those of you who are ambitious.
This tutorial assumes several things about the reader:

Quoted Strings
 
1 
2 
3 
4 
 		 <?php
$str_1 = 'I am a string in single quotes';
$str_2 = "I am a string in double quotes";
?>

The easiest way to specify a simple string is to enclose it in single quotes (the character '). Enclosing the string in double-quotes ("), will allow PHP to understand additional escape sequences for special characters:
\n linefeed (LF or 0x0A (10) in ASCII)
\r carriage return (CR or 0x0D (13) in ASCII)
\t horizontal tab (HT or 0x09 (9) in ASCII)
\\ backslash
\$ dollar sign
\" double-quote
\[0-7]{1,3} the sequence of characters matching the regular expression is a character in octal notation
\x[0-9A-Fa-f]{1,2} the sequence of characters matching the regular expression is a character in hexadecimal notation
The PHP parser will determine string start and end points based on matching quotes. That means every string must start and end with the same type of quote. This is the cause of many headaches for some coders. You can include single quote marks and other characters in the double quoted string, which often lends confusion to when you have actually closed your string.
Here's a trick I use to ensure I avoid this problem... When I create a string variable, I complete the entire declaration statement prior to filling in a value:

1 
2 
3 
4 
 		 <?php
$str_1 = '';
$str_2 = "";
?>

Then follow through by adding my value to the statement:

1 
2 
3 
4 
 		 <?php
$str_1 = 'I am a string in single quotes';
$str_2 = "I am a string in double quotes";
?>

Escape Characters
To include a quote in the middle of a string and have the parser ignore it you need to escape it with a backslash. This tells PHP that the quote should not be used as the end point of the string, and to continue to the next quote:

1 
2 
3 
4 
5 
6 
 		 <?php
$str_1 = 'I am a \'string\' in single quotes';
echo $str_1."<br>";
$str_2 = "I am a \"string\" in double quotes";
echo $str_2."<br>";
?>

To include a backslash itself in a string, you need to escape the backslash with a backslash:

1 
2 
3 
4 
5 
6 
 		 <?php
$str_1 = 'I am a \\backslash\\ in single quotes';
echo $str_1."<br>";
$str_2 = "I am a \\backslash\\ in double quotes";
echo $str_2."<br>";
?>

Another way to delimit strings is by using heredoc syntax ("<<<"). Using this syntax a unique identifier is placed after <<<, the string to be stored or displayed, and finally the same identifier to close the quotation.
The closing identifier must begin in the first column of the line. Also, the identifier used must follow the same naming rules as any other label in PHP: it must contain only alphanumeric characters and underscores, and must start with a non-digit character or underscore.

1 
2 
3 
4 
5 
6 
7 
 		 <?php
$str = <<<EOD
Example of a string
spanning multiple lines
using the heredoc syntax.
EOD;
?>

The heredoc system is also known as the Here Document format. This system is normally used for large amounts of text, with no worries of overlapping quotes. You do not need to escape quotes in your here docs, but you can still use escape codes.

Concatenating Strings
Using the concatenate operator (.), strings can be easily joined together.

1 
2 
3 
4 
5 
6 
7 
 		 <?php
$str_1 = 'This';
$str_2 = 'is a';
$str_3 = 'string.';
$str = $str_1. " " .$str_2. " " .$str_3;
echo $str;
?>

Used with the assignment operator (=), you can append data to your string - making the creation of large strings very simple.

1 
2 
3 
4 
5 
6 
 		 <?php
$str = 'This ';
$str .= 'is a ';
$str .= 'string.';
echo $str;
?>

Variables & Strings
PHP lets you include variables inside double quoted strings:

1 
2 
3 
4 
5 
6 
 		 <?php
$str_1 = 'This is a';
$str_2 = 'string.';
$str = "$str_1 $str_2";
echo $str;
?>

Double and single quoted strings containing variables will not give you the same results. Double quoted strings will interpret the variable while single quoted strings will treat it as part of the original string variable:

1 
2 
3 
4 
5 
6 
7 
 		 <?php
$num = 9;
echo 'Display the number $num';
// displays "Display the number $num" to the screen
echo "Display the number $num";
// displays "Display the number 9" to the screen
?>

Because scripts using single quotes run slightly faster, double quotes should only be used when you want to include special escape characters or variables within the string.
It is advisable to concatenate strings, as shown in prior examples, when you are using complex or even simple variable substitution.

1 
2 
3 
4 
5 
6 
 		 <?php
$str_1 = 'This is a';
$str_2 = 'nother boring';
$str_3 = 'string example';
echo $str_1.$str_2. " " .$str_3. ".";
?>

Conclusion
When working with strings, don't make the PHP interpreter have to work any harder than it already is. A large portion of web development involves handling and generating strings, knowing how to properly handle and manipulate strings is important. To maximize your code, correct string definition is the first, and often forgotten, place to start.

Always remember to use single quotes to enclose your strings, unless including variables within the string variable. And when you need to involve variables, try to make use of concatenation to avoid using double quotes wherever possible. Your end result will be a much faster site.