defining a var happens whenever you assign a value to it.
$sName = 'John';
But sometimes you cannot assing a value to all your vars. For example, when you use and HTML form to post data to your script, you want to use the values that come from the form, so you don't want to overwrite those with anything.
In those cases you should first check to see if the variable you want to use actually exists using the 'isset()' function:
if (!isset($sName))
{
echo 'sName not set';
}
else
{
echo 'Your name is:'.$sName;
};
