redmine_rest_api_create_issue.php

<?php
$client = new \GuzzleHttp\Client();
$res = $client->post(
    'https://hoge/issues.json',
    [
        'json' => [
            'key' => $api_key
            ,'issue' => [
                'project_id'    => 1
                ,'subject'      => '題名サンプル'
                ,'priority_id'  => 3
            ]
        ]
    ]
);