I just realized that you can't just use an if statement on a function, for example this doesn't work:

function sayHello()
{
    echo "Hello World";
}

if(sayHello())
    echo "Function Worked";
else
    echo "Function Failed";

I also saw that a function can't be put as the value of a variable. So how can I do an if statement to check if a function has executed properly and display it to the browser?