<input value='<?php if(isset($var)){print($var);}; ?>'>

<input value='<?php printvar('myvar'); ?>'>

function issetor(&$var, $default = false) {
    return isset($var) ? $var : $default;
}

echo issetor($myVar);

$user = issetor($_GET['user'], 'guest');

$user = $_GET['user'] ?? 'guest';

<?= $myVar ?? '' ?>

<input value="<?php echo isset($var) ? $var : '' ?>">

<input value='<?php @print($var); ?>'>

function printvar($name) {
    if(isset($GLOBALS[$name])) echo $GLOBALS[$name];
}

function printvar(&$variable)
{
    if (isset($variable)) {
        echo $variable;
    }
}

<?= isset($my_variable) ? $my_variable : $my_variable="Some Value"; ?>

<input value='<?php printvar(&$myvar); ?>' />

function printvar(&$val) {
    if (isset($val)) echo $val;
}

$var;
(isset($var)) ? (print $var) : '';

$var = 'hello var';
(isset($var)) ? (print $var) : '';