You're allowed to return one and only one value back from functions, and you do this by using the return statement. In our example, we could have used "return 'foo';" or "return 10 + 10;" to pass other values back, but "return 1;" is easiest, and usually the most common as it is the same as "return true;"
You can return any variable you want, as long as it is just one variable - it can be an integer, a string, a database connection, etc. The "return" keyword sets up the function return value to be whatever variable you use with it, then exits the function immediately. You can also just use "return;", which means "exit without sending a value back."
That will output "In function", followed by "1", and then the script will terminate. The reason we never see "Leaving function..." is because the line "return 1" passes one back then immediately exits - the second print statement in foo() is never reached.
If you want to pass more than one value back, you need to use an array - this is covered soon.
A popular thing to do is to return the value of a conditional statement, e.g.:
If $i is indeed greater than 10, the > operator will return 1, so it is the same as having "return 1", but if $i is less than or equal to ten, it is the same as being "return 0".