$re = '/(<FO_KEP>[^<]*<\/FO_KEP>)\r?\n(<OSSZESZERELESI_KEP>[^<]*<\/OSSZESZERELESI_KEP>)/';
$str = '<TERMEK>
<TERMEK_KOD>01008213</TERMEK_KOD>
<FO_KEP>https://www.temponabytok.sk/Files/obrazky/01008213.jpg</FO_KEP>
<OSSZESZERELESI_KEP>https://www.temponabytok.sk/Files/dokumenty/Vratko_montazny_navod.jpg</OSSZESZERELESI_KEP>
<OBRAZKY>https://www.temponabytok.sk/Files/2341000/2341428/245320712.jpg</OBRAZKY>
</TERMEK>';
$subst = "\2\n\1";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php