$re = '/<img.*src="(.*\)/i';
$str = '<img style="gdgdF" src="http://multimedia.pol.dk/archive/00802/RB_PLUS_Danskerne___802815p.jpg" width="369" height="253" alt="SPRITKONTROL" />';
$subst = "<img src=\"http://multimedia.pol.dk/archive/00802/SNOOTS.jpg\"";
$result = preg_replace($re, $subst, $str, 1);
echo "The result of the substitution is ".$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