easy way to execute conditional html / javascript / css / other language code with php if else:

<?php if (condition): ?>

html code to run if condition is true

<?php else: ?>

html code to run if condition is false

<?php endif ?>

Left-to-right evaluation of && operators has one useful feature: evaluation stops after first "false" operand is encountered.
 
This feature can be useful for creating following construction:

$someVar==123 is not evaluated, so there will be no warnings such as "Undefined variable $someVar":
<?php
// $someVar=123; - commented line
if ((!empty($someVar))&&($someVar==123))
{
    echo $someVar;
}
?>

Function someFunc($someVar) will not be called:
<?php
// $someVar=123; - commented line
if ((!empty($someVar))&&(someFunc($someVar)))
{
    echo $someVar;
}
?>

This will give "Warning: Undefined variable $someVar" error. Order matters:
<?php
// $someVar=123;
if ((someFunc($someVar))&&(!empty($someVar)))
{
    echo $someVar;
}
?>

You can have 'nested' if statements withing a single if statement, using additional parenthesis.
For example, instead of having:

<?php
if( $a == 1 || $a == 2 ) {
    if( $b == 3 || $b == 4 ) {
        if( $c == 5 || $ d == 6 ) {
             //Do something here.
        }
    }
}
?>

You could just simply do this:

<?php
if( ($a==1 || $a==2) && ($b==3 || $b==4) && ($c==5 || $c==6) ) {
    //do that something here.
}
?>

Hope this helps!

re: #80305

Again useful for newbies:

if you need to compare a variable with a value, instead of doing

<?php
if ($foo == 3) bar();
?>

do

<?php
if (3 == $foo) bar();
?>

this way, if you forget a =, it will become

<?php
if (3 = $foo) bar();
?>

and PHP will report an error.

An other way for controls is the ternary operator (see Comparison Operators) that can be used as follows:

<?php
$v = 1;

$r = (1 == $v) ? 'Yes' : 'No'; // $r is set to 'Yes'
$r = (3 == $v) ? 'Yes' : 'No'; // $r is set to 'No'

echo (1 == $v) ? 'Yes' : 'No'; // 'Yes' will be printed

// and since PHP 5.3
$v = 'My Value';
$r = ($v) ?: 'No Value'; // $r is set to 'My Value' because $v is evaluated to TRUE

$v = '';
echo ($v) ?: 'No Value'; // 'No Value' will be printed because $v is evaluated to FALSE
?>

Parentheses can be left out in all examples above.