mysql_variable_test.php

<?php
$sql = <<<EOF
SET @ship_id = (SELECT ship_id FROM orders WHERE ship_id IS NOT NULL ORDER BY ship_id LIMIT 1);
SELECT 
	id, 
	user_id, 
	COALESCE(ship_id, @ship_id) AS ship_id
FROM 
	orders;
EOF;

$servername = "localhost";
$username = "root";
$password = "root";
$dbname = "test";

try {
    $conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    $stmt = $conn->prepare($sql); 
    $stmt->execute();

    // set the resulting array to associative
    $result = $stmt->setFetchMode(PDO::FETCH_ASSOC); 
    foreach($stmt->fetchAll() as $k=>$v) { 
        print_r($v);
    }
}
catch(PDOException $e) {
    echo "Error: " . $e->getMessage();
}
$conn = null;

echo PHP_EOL;
?>