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from:http://www.smarty.net/manual/en/language.function.if.php

{if},{elseif},{else}

{if} statements in Smarty have much the same flexibility as PHP if statements, with a few added features for the template engine. Every {if} must be paired with a matching {/if} . {else} and {elseif} are also permitted. All PHP conditionals and functions are recognized, such as || , or , && , and , is_array() , etc.

If $security is enabled, only PHP functions from the IF_FUNCS array from $security_settings are allowed.

The following is a list of recognized qualifiers, which must be separated from surrounding elements by spaces. Note that items listed in [brackets] are optional. PHP equivalents are shown where applicable.

Qualifier	Alternates	Syntax Example	Meaning	PHP Equivalent
==	eq	$a eq $b	equals	==
!=	ne, neq	$a neq $b	not equals	!=
>	gt	$a gt $b	greater than	>
<	lt	$a lt $b	less than	<
>=	gte, ge	$a ge $b	greater than or equal	>=
<=	lte, le	$a le $b	less than or equal	<=
===		$a === 0	check for identity	===
!	not	not $a	negation (unary)	!
%	mod	$a mod $b	modulous	%
is [not] div by		$a is not div by 4	divisible by	$a % $b == 0
is [not] even		$a is not even	[not] an even number (unary)	$a % 2 == 0
is [not] even by		$a is not even by $b	grouping level [not] even	($a / $b) % 2 == 0
is [not] odd		$a is not odd	[not] an odd number (unary)	$a % 2 != 0
is [not] odd by		$a is not odd by $b	[not] an odd grouping	($a / $b) % 2 != 0

Example 7-15. {if} statements

{if $name eq 'Fred'}
    Welcome Sir.
{elseif $name eq 'Wilma'}
    Welcome Ma'am.
{else}
    Welcome, whatever you are.
{/if}

{* an example with "or" logic *}
{if $name eq 'Fred' or $name eq 'Wilma'}
   ...
{/if}

{* same as above *}
{if $name == 'Fred' || $name == 'Wilma'}
   ...
{/if}


{* parenthesis are allowed *}
{if ( $amount < 0 or $amount > 1000 ) and $volume >= #minVolAmt#}
   ...
{/if}


{* you can also embed php function calls *}
{if count($var) gt 0}
   ...
{/if}

{* check for array. *}
{if is_array($foo) }
   .....
{/if}

{* check for not null. *}
{if isset($foo) }
   .....
{/if}


{* test if values are even or odd *}
{if $var is even}
   ...
{/if}
{if $var is odd}
   ...
{/if}
{if $var is not odd}
   ...
{/if}


{* test if var is divisible by 4 *}
{if $var is div by 4}
   ...
{/if}


{*
  test if var is even, grouped by two. i.e.,
  0=even, 1=even, 2=odd, 3=odd, 4=even, 5=even, etc.
*}
{if $var is even by 2}
   ...
{/if}

{* 0=even, 1=even, 2=even, 3=odd, 4=odd, 5=odd, etc. *}
{if $var is even by 3}
   ...
{/if}

Example 7-16. {if} with more examples

{if isset($name) && $name == 'Blog'}
     {* do something *}
{elseif $name == $foo}
    {* do something *}
{/if}

{if is_array($foo) && count($foo) > 0)
    {* do a foreach loop *}
{/if}