Simple: How to replace “all between” with php? [duplicate]

Answer 1

$string = "<tag>I do not know what is here</tag>";
$new_text = 'I know now'; 
echo preg_replace('#(<tag.*?>).*?(</tag>)#', '$1'.$new_text.'$2' , $string); //<tag>I know now</tag>

Answer 2

If "tag" changes:

$string = "<tag>i dont know what is here</tag>";
$string = preg_replace('|^<([a-z]*).*|', '<$1></$1>', $string)
echo $string; // <tag></tag>