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Heredoc is one of the ways to store or print a block of text in PHP. The data stored in the heredoc variable is more readable and error-free than other variables for using indentation and newline.
A heredoc is a way to define a multiline string, while maintaining the original indentation & formatting. You can use a Heredoc to embed snippets of SQL, HTML, or even XML code. A Heredoc starts with <<- , followed by a word that represents the name for the heredoc, then the contents.
Heredoc and nowdoc provide useful alternatives to defining strings in PHP to the more widely used quoted string syntax. They are especially useful when we need to define a string that spans multiple lines (new lines are also interpreted when used in quoted strings) and where use of whitespace is important.
In computing, a here document (here-document, here-text, heredoc, hereis, here-string or here-script) is a file literal or input stream literal: it is a section of a source code file that is treated as if it were a separate file.
The heredoc syntax is much cleaner to me and it is really useful for multi-line strings and avoiding quoting issues. Back in the day I used to use them to construct SQL queries:
$sql = <<<SQL
select *
from $tablename
where id in [$order_ids_list]
and product_name = "widgets"
SQL;
To me this has a lower probability of introducing a syntax error than using quotes:
$sql = "
select *
from $tablename
where id in [$order_ids_list]
and product_name = \"widgets\"
";
Another point is to avoid escaping double quotes in your string:
$x = "The point of the \"argument" was to illustrate the use of here documents";
The problem with the above is the syntax error (the missing escaped quote) I just introduced as opposed to here document syntax:
$x = <<<EOF
The point of the "argument" was to illustrate the use of here documents
EOF;
It is a bit of style, but I use the following as rules for single, double and here documents for defining strings:
'no variables here'
"Today is ${user}'s birthday"
Heredoc's are a great alternative to quoted strings because of increased readability and maintainability. You don't have to escape quotes and (good) IDEs or text editors will use the proper syntax highlighting.
A very common example: echoing out HTML from within PHP:
$html = <<<HTML
<div class='something'>
<ul class='mylist'>
<li>$something</li>
<li>$whatever</li>
<li>$testing123</li>
</ul>
</div>
HTML;
// Sometime later
echo $html;
It is easy to read and easy to maintain.
The alternative is echoing quoted strings, which end up containing escaped quotes and IDEs aren't going to highlight the syntax for that language, which leads to poor readability and more difficulty in maintenance.
Updated answer for Your Common Sense
Of course you wouldn't want to see an SQL query highlighted as HTML. To use other languages, simply change the language in the syntax:
$sql = <<<SQL
SELECT * FROM table
SQL;
Some IDEs highlight the code in heredoc strings automatically - which makes using heredoc for XML or HTML visually appealing.
I personally like it for longer parts of i.e. XML since I don't have to care about quoting quote characters and can simply paste the XML.
First of all, all the reasons are subjective. It's more like a matter of taste rather than a reason.
Personally, I find heredoc quite useless and use it occasionally, most of the time when I need to get some HTML into a variable and don't want to bother with output buffering, to form an HTML email message for example.
Formatting doesn't fit general indentation rules, but I don't think it's a big deal.
//some code at it's proper level
$this->body = <<<HERE
heredoc text sticks to the left border
but it seems OK to me.
HERE;
$this->title = "Feedback";
//and so on
As for the examples in the accepted answer, it is merely cheating.
String examples, in fact, being more concise if one won't cheat on them
$sql = "SELECT * FROM $tablename
WHERE id in [$order_ids_list]
AND product_name = 'widgets'";
$x = 'The point of the "argument" was to illustrate the use of here documents';
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