Isset

issetReturns true if the variable(s) passed to it are different from null.

If multiple parameters are supplied then isset() will return true only if all of the parameters are not null.

<?php
if(isset(mixed $var))
{
	echo "SET";
}else{
	echo "Not SET";
}

{* Using | to check variable set or not*}
{if $foo|isset}SET{else}not set or null{/if}

{* Using php function style to check variable set or not*}
{if isset($foo)}SET{else}not set or null{/if}

{* Assign 1 to $foo variable*}
{$foo=1}

{* Using | to check variable set or not*}
{if $foo|isset}SET{else}not set or null{/if}

{$bar=null}
{if isset($bar)}SET{else}not set or null{/if}

Output

not set or null
not set or null
SET
not set or null

Note that, doing:

{if $foo}{$foo}{/if}

..is the same as if you used:
{if $foo|isset}SET{else}not set or null{/if}

OR

{if isset($foo)}SET{else}not set or null{/if}