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How to use %S in PHP, HTML or XML

with printf or sprintf characters preceded by the % sign are placeholders (or tokens). They will be replaced by a variable passed as an argument. 
 
Example:
$str1 = 'best';
$str2 = 'world';

$say = sprintf('Tivie is the %s in the %s!', $str1, $str2);
echo $say;
 
This will output:
Tivie is the best in the world!
Note: There are more placeholders (%s for string, %d for dec number, etc...)

Order:
The order in which you pass the arguments counts. If you switch $str1 with $str2 as
$say = sprintf('Tivie is the %s in the %s!', $str2, $str1); 
it will print
"Tivie is the world in the best!"
You can, however, change the reading order of arguments like this:
$say = sprintf('Tivie is the %2$s in the %1$s!', $str2, $str1);
which will print the sentence correctly.

Also, keep in mind that PHP is a dynamic language and does not require (or support) explicit type definition. That means it juggles variable types as needed. In sprint it means that if you pass a "string" as argument for a number placeholder (%d), that string will be converted to a number (int, float...) which can have strange results. Here's an example:

$onevar = 2;
$anothervar = 'pocket';
$say = sprintf('I have %d chocolate(s) in my %d.', $onevar, $anothervar);
echo $say;
 
this will print
I have 2 chocolate(s) in my 0.
More reading at PHPdocs