In expression (expr1) ? (expr2) : (expr3), if not set expre2 and expr1 is true then print 1 when echo usage.

isset function.php

<?php
$var = "string";

echo isset($var) ? : 'ooh'; // 1

//echo isset($var) ? : ; // Parse error: syntax error, unexpected ';'

var_dump(isset($var) ? : 'ooh'); // boolean true