String in PHP

A string is series of characters, where a character is the same as a byte. This means that PHP only supports a 256-character set, and hence does not offer native Unicode support. See details of the string type.

A string literal can be specified in four different ways:

1.single quoted

2.double quoted

3.heredoc syntax

4.nowdoc syntax 

Define  String With single quoted & double quoted
<?php 
$t1="My Name is Virendra<br/>";
$t2="My Name is Virendra";
echo $t1;
echo $t2;

?>

Difference between single quote and double quote string in php.
A single-quoted string does not have variables within it interpreted. A double-quoted string does.

The single-quoted strings are faster at runtime because they do not need to be parsed.

Define  String heredoc syntax quoted & nowdoc syntax
As you see the heredoc starts with the <<< operator and an identifier. 
<?php
   $str = <<<DEMO
    This is a
    demo message
   with heredoc.
   DEMO; 
   echo $str;

?>
Nowdoc syntax is similar to heredoc. The only difference is that the delimiter is enclosed within single quotes:
Syntax:
$myString =  <<< 'DELIMITER' 
         (insert string here) 

DELIMITER;  
Traditionally, heredoc & nowdoc delimiters are written in uppercase, like constants.