RE: IF documentation

Dear Sir, i am new to PHP and am a arts major. I was curious to know in the example provided in the if statement <?php if ($a > $b) echo "a is bigger than b"; ?> website link: http://php.net/manual/en/control-structures.if.php The above example doesn't work unless it is preceded by defining values for $a and $b. Is it possible to show on the documentation the preceding values so that readers don't assume it will work without defining $a and $b. The above example only works if $a and $b are defined I think. Regards Var