If null use other variable in one line in PHP

you can do echo $test ?: 'hello';

This will echo $test if it is true and 'hello' otherwise.

Note it will throw a notice or strict error if $test is not set but...

This shouldn't be a problem since most servers are set to ignore these errors. Most frameworks have code that triggers these errors.

Edit: This is a classic Ternary Operator, but with the middle part left out. Available since PHP 5.3.

echo $test ? $test : 'hello'; // this is the same
echo $test ?: 'hello';        // as this one

This only checks for the truthiness of the first variable and not if it is undefined, in which case it triggers the E_NOTICE error. For the latter, check the PHP7 answer below (soon hopefully above).

From PHP 7 onwards you can use something called a coalesce operator which does exactly what you want without the E_NOTICE that ?: triggers.

To use it you use ?? which will check if the value on the left is set and not null.

$arr = array($one ?? 'one?', $two ?? 'two?'); 

See @Yamiko's answer below for a PHP7 solution https://stackoverflow.com/a/29217577/140413

 echo (!$test) ? 'hello' : $test;

Or you can be a little more robust and do this

echo isset($test) ? $test : 'hello'; 

One-liner. Super readable, works for regular variables, arrays and objects.

// standard variable string
$result = @$var_str ?: "default";

// missing array element
$result = @$var_arr["missing"] ?: "default";

// missing object member
$result = @$var_obj->missing ?: "default";

See it in action: Php Sandbox Demo