The most simple method to post a message to slack via an 'incoming webhook' from PHP

a-stylized-message.php

<?php

function post_slack_message($msg) {
    $opts = [
        'http' => [
            'method'  => 'POST',
            'header' => "Content-type: application/json\r\n",
            'content' => json_encode([
                'attachments' => [
                    [
                        'color' => '#ff0000',
                        'blocks' => [
                            [
                                'type' => 'header',
                                'text' => [
                                    'type' => 'plain_text',
                                    'text' => $msg,
                                ]
                            ]
                        ]
                    ],
                ]
            ]),
            'ignore_errors' => true,
        ]
    ];

    $response = file_get_contents(getenv('SLACK_WEBHOOK_URL'), false, stream_context_create($opts));
    if($response !== 'ok') {
        var_dump($http_response_header); // dump headers
        var_dump($response); // dump body as well (usually where the problem can be found)
        throw new \Exception("Unable to post to slack");
    }

}

post_slack_message('hi there');

just-a-plain-text-message.php

<?php

function post_slack_message_styled($msg) {
    $opts = [
        'http' => [
            'method'  => 'POST',
            'header' => "Content-type: application/json\r\n",
            'content' => json_encode([
                'text' => $msg,
            ]),
            'ignore_errors' => true,
        ]
    ];

    $response = file_get_contents(getenv('SLACK_WEBHOOK_URL'), false, stream_context_create($opts));
    if($response !== 'ok') {
        var_dump($http_response_header); // dump headers
        var_dump($response); // dump body as well (usually where the problem can be found)
        throw new \Exception("Unable to post to slack");
    }

}

post_slack_message_styled('hi there');