Explain the differences between echo, print and printf?

Echo just outputs the contents following the construct to the screen.
print is also a construct (so parentheses are optional when calling it), but it returns TRUE on successful output and FALSE if it was unable to print out the string.
However, you can pass multiple parameters to echo, like:

echo 'Welcome ', 'to', ' ', 'php mysql guide!';

and it will output the string "Welcome to php mysql guide!"
print does not take multiple parameters.
It is also generally argued that echo is faster, but usually the speed advantage is negligible, and might not be there for future versions of PHP.
printf is a function, not a construct, and allows such advantages as formatted output, but it’s the slowest way to print out data out of echo, print and printf.