It's the start of a heredoc. you can do:
$data = <<< _END
You can write anything you want in between the start and end
_END;
_END can be just about anything. You could put EOF or STUFF. as long as you use the same thing at the start and the finish.
This signifies the beginning of a heredoc (a multi-line string that allows you to use quotation marks in the middle, unescaped) that ends when you encounter the _END
It can be useful to define HTML in one of these if the goal is to assign it to a variable or pass it to a function rather than printing it to the web server immediately.
Heredoc syntax:
A third way to delimit strings is the heredoc syntax:
<<<. After this operator, an identifier is provided, then a newline. The string itself follows, and then the same identifier again to close the quotation.The closing identifier must begin in the first column of the line. Also, the identifier 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.
Warning It is very important to note that the line with the closing identifier must contain no other characters, except a semicolon (;). That means especially that the identifier may not be indented, and there may not be any spaces or tabs before or after the semicolon. It's also important to realize that the first character before the closing identifier must be a newline as defined by the local operating system. This is \n on UNIX systems, including Mac OS X. The closing delimiter must also be followed by a newline.
If this rule is broken and the closing identifier is not "clean", it will not be considered a closing identifier, and PHP will continue looking for one. If a proper closing identifier is not found before the end of the current file, a parse error will result at the last line.
Heredocs can not be used for initializing class properties. Since PHP 5.3, this limitation is valid only for heredocs containing variables...
Read it here: http://php.net/manual/en/language.types.string.php#language.types.string.syntax.heredoc
This is well documented. From the PHP Manual:
The closing tag of a PHP block at the end of a file is optional, and in some cases omitting it is helpful when using include() or require(), so unwanted whitespace will not occur at the end of files, and you will still be able to add headers to the response later. It is also handy if you use output buffering, and would not like to see added unwanted whitespace at the end of the parts generated by the included files.
Omitting the closing tag helps you prevent accidental whitespace or newlines from being added to the end of the file.
That's a core PHP feature: unlike other languages, you need to tag PHP code with a special tag (normally <?php) because everything else is considered literal output:
This is not PHP
<?php
echo 'This is PHP' . PHP_EOL;
?>
This is not PHP either
D:\tmp>php test.php
This is not PHP
This is PHP
This is not PHP either
Although the manual mentions HTML, PHP doesn't really know/care what content-type is outside its tags.
If you forget to close a PHP block when further stuff follows you normally get a syntax error:
This is not PHP
<?php
echo 'This is PHP' . PHP_EOL;
This is not PHP either
D:\tmp>php test.php
PHP Parse error: syntax error, unexpected 'is' (T_STRING) in D:\tmp\borrame.php on line 6
Blank lines are a sort of special case because they are valid and almost invisible in almost all languages (PHP, HTML, CSS, JavaScript...) so they often unnoticed.
Once you've removed the ?> tag, your literal blank lines have disappeared from the script output because they've become part of the PHP code (and, as such, they've started to get ignored).
Of course, blank lines are ignored by PHP but not necessarily by whatever you are generating which, as I said, does not need to be HTML: it can be a picture, a PDF document, an Excel spreadsheet. Bogus white lines can be easily avoided by not closing the last PHP block when it's the last part of the file.