Find the img tags in a text

I have a random text:

$s = " eu sou campeao <img src='leo.jpg' /> teste lala  ";

I need to add the path of the image and look like this:

$s = " eu sou campeao <img src='img/leo.jpg' /> teste lala  ";

Note: I need to add "img /" inside the src directory. There may come several images, but I'll add the same string.