in the same str_replace line… you will see the 2nd argument is ”… just change it to ‘my new string’…
It works 
Thank you very much This problem has been solved.. The other was that all messages users receive from the forum have has sender “WordPress” and not the name of my website. I’ am reffering also to messages that inform users of a new PM or new posts. If I receive an email from this forum the sender is simple:press support and not wordpress.. what could I do ??? Thank you for your help
that is how wp send emails… there are smtp plugins out there that let you set who you want the emails to be from… for sp emails, you can go to forum – options – email settings and configure it too.. but that wont affect normal wp emails like registration… they will still use the wp default stuff…