Unknown modifier 't'

Alternatively, you can use different pattern delimiters:
$url_query = preg_replace('#(.*?)/strona-([0-9]*)-#', '$1/strona-'.$counter.'-', $targetpage);

This function can sometimes be helpful.  
http://us3.php.net/manual/en/function.preg-quote.php

But that said, the choice of the regex delimiter is usually made in such a way that it does not appear in the regex pattern.

See the code snippet for an explanation of what the regex engine is looking at in the line of code.

When you are developing regular expressions (or for that matter any complicated piece of code) a test-driven design is a really useful process.  You can see the thought process that goes into TDD in the article here.
http://www.experts-exchange.com/Web_Development/Web_Languages-Standards/PHP/A_7830-A-Quick-Tour-of-Test-Driven-Development.html

$regex = '/(.*?)/strona-([0-9]*)-/';

$regex
= '/'        // REGEX DELIMITER
. '(.*?)'    // A GROUP OF ANYTHING OR NOTHING
. '/'        // REGEX DELIMITER (OOPS)
. 'strona'   // A STRING, ETC... BUT 's' IS BEING TREATED AS A MODIFIER SINCE IT COMES AFTER THE REGEX DELIMITER.

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