Add a string in the middle of another string?

I have a string into variable that change often except for a couple of constannt word it contains...

Is it a fonction to search into the string for a "specific word", then append another string without crunching the rest of the initial string?

I know it seems easy to do this, but I can't find any example that works...

Thanks..

this might be what you are looking for...

you have a variable like this:
$variable = "you can do this";

you then want to add the following string to the above variable after the word "can";
$add = "not";

so try this function:
str_replace("can", "can$add", $variable);

this should yeild the following:
"you cannot do this"

well... I think too much...

Thanks, this is the way I will do it...