What does %S mean 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

In printf, %s is a placeholder for data that will be inserted into the string. The extra arguments to printf are the values to be inserted. They get associated with the placeholders positionally: the first placeholder gets the first value, the second the second value, and so on.

%s is a type specifier which will be replaced to valuable’s value (string) in case of %s.

Besides %s you can use other specifiers, most popular are below:

d – the argument is treated as an integer, and presented as a (signed) decimal number.

f – the argument is treated as a float, and presented as a floating-point number (locale
aware).

s – the argument is treated as and presented as a string.

$num = 5; 
$location = 'tree';

$format = 'There are %d monkeys in the %s'; 
echo sprintf($format, $num, $location); 

Will output: “There are 5 monkeys in the tree.”

The printf() or sprintf() function writes a formatted string to a variable.
Here is the Syntax:

sprintf(format,arg1,arg2,arg++)

format:

• %% – Returns a percent sign
• %b – Binary number
• %c – The character according to the ASCII value
• %d – Signed decimal number (negative, zero or positive)
• %e – Scientific notation using a lowercase (e.g. 1.2e+2)
• %E – Scientific notation using a uppercase (e.g. 1.2E+2)
• %u – Unsigned decimal number (equal to or greater than zero)
• %f – Floating-point number (local settings aware)
• %F – Floating-point number (not local settings aware)
• %g – shorter of %e and %f
• %G – shorter of %E and %f
• %o – Octal number
• %s – String
• %x – Hexadecimal number (lowercase letters)
• %X – Hexadecimal number (uppercase letters)

arg1:

• The argument to be inserted at the first %-sign in the format
  string..(Required.)

arg2:

• The argument to be inserted at the second %-sign in the format
  string. (Optional)

arg++:

• The argument to be inserted at the third, fourth, etc. %-sign in
  the format string (Optional)

Example 1:

$number = 9;
$str = "New York";
$txt = sprintf("There are approximately %u million people in %s.",$number,$str);
echo $txt;

This will output:

There are approximately 9 million people in New York.

The arg1, arg2, arg++ parameters will be inserted at percent (%) signs in the main string. This function works “step-by-step”. At the first % sign, arg1 is inserted, at the second % sign, arg2 is inserted, etc.

Note: If there are more % signs than arguments, you must use
placeholders. A placeholder is inserted after the % sign, and consists
of the argument- number and “\$”. Let see another Example:

Example 2

$number = 123;
$txt = sprintf("With 2 decimals: %1$.2f
<br>With no decimals: %1$u",$number);
echo $txt;

This will output:

With 2 decimals: 123.00
With no decimals: 123

Another important tip to remember is that:

With printf() and sprintf() functions, escape character is not
backslash ‘\’ but rather ‘%’. Ie. to print ‘%’ character you need to
escape it with itself:

printf('%%%s%%', 'Nigeria Naira');

This will output:

%Nigeria Naira%

Feel free to explore the official PHP Documentation