DROP DATABASE IF EXISTS `phpdb`;
CREATE DATABASE `phpdb`;

USE `phpdb`;

DROP PROCEDURE IF EXISTS demoSp;
DELIMITER $$
CREATE PROCEDURE demoSp(IN inputParam VARCHAR(255), INOUT inOutParam INT)
BEGIN
    DECLARE z INT;
    SET z = inOutParam + 1;
    SET inOutParam = z;

    SELECT inputParam;

    SELECT CONCAT('zyxw', inputParam);
END;$$
DELIMITER ;

USE `phpdb`;
CALL `demoSp`('abcdefg', @inOutParam);

<?php

$host = 'localhost';
$username = 'root';
$password = '';
$dbname = 'phpdb';

try {
    $pdo = new PDO("mysql:host=$host;dbname=$dbname", $username, $password); ##
    $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    echo "Connected to MySQL Server successfully." . "n";
    $sql = "CALL demoSp('abcdefg', @inOutParam);";
    $rows = $pdo->query($sql);
    foreach($rows as $row) {
      echo $row[0] . "n";
    }
} catch (Exception $e) {
    $error = $e->getMessage();
    echo $error . "n";
} finally {
    $pdo = null;
    echo "Connection closed." . "n";
}