Concatenate output of php function into string

Question

I am looping through my custom post types and creating a table inside the string. Once the loop is done I will echo the string so it will show the actual table.

Now I want to add a delete button for each custom post type, for this, I am using the Wordpress built-in function settings_fields('fahad_plugin_cpt_settings'); and submit_button('Delete'); This function is generating some HTML content which I want to store inside the table string I am creating.

Now the problem is the output of those methods are not being concatenated inside the string but generated outside the table string.

Here is my code

<?php
                $options = get_option('fahad_plugin_cpt') ?: array();

                $data = '';
                $data .= '<table class="table">';
                $data .= '<thead>';
                $data .= '<tr>';
                $data .= '<th>ID</th>';
                $data .= '<th>Singular Name</th>';
                $data .= '<th>Plural Name</th>';
                $data .= '<th>Public</th>';
                $data .= '<th>Archive</th>';
                $data .= '<th>Actions</th>';
                $data .= '</tr>';
                $data .= '</thead>';

                $data .= '<tbody>';
                foreach ($options as $option)
                    {
                        $public = isset($option['public']) ? 'Yes' : 'No';
                        $has_archive = isset($option['has_archive']) ? 'Yes' : 'No';

                        $data .= '<tr>';
                        $data .= '<td>'.$option['post_type'].'</td>';
                        $data .= '<td>'.$option['singular_name'].'</td>';
                        $data .= '<td>'.$option['plural_name'].'</td>';
                        $data .= '<td>'.$public.'</td>';
                        $data .= '<td>'.$has_archive.'</td>';
                        $data .= '<td>';
                        //$data .= '<button type="button" class="btn btn-success btn-sm">Edit</button>';
                        $data .= '<form method="post" action="options.php">';
                        $data .= settings_fields('fahad_plugin_cpt_settings');
                        $data .= submit_button('Delete', 'btn btn-danger btn-sm', 'submit', false);
                        $data .= '</form>';
                        $data .= '</td>';
                        $data .= '</tr>';
                    }
                $data .= '</tbody>';
                $data .= '</table>';


            echo $data
            ?>

Actual String

'<table class="table">
<thead>
    <tr>
        <th>ID</th>
        <th>Singular Name</th>
        <th>Plural Name</th>
        <th>Public</th>
        <th>Archive</th>
        <th>Actions</th>
    </tr>
</thead>
<tbody>
    <tr>
        <td>comicbook</td>
        <td>Comic Book</td>
        <td>Comic Books</td>
        <td>Yes</td>
        <td>Yes</td>
        <td>
            <form method="post" action="options.php"></form>
        </td>
    </tr>
    <tr>
        <td>product</td>
        <td>Product</td>
        <td>Products</td>
        <td>No</td>
        <td>No</td>
        <td>
            <form method="post" action="options.php"></form>
        </td>
    </tr>
</tbody>

'

Output that should be concatenated inside the string

<input type="hidden" name="option_page" value="fahad_plugin_cpt_settings">
<input type="hidden" name="action" value="update">
<input type="hidden" id="_wpnonce" name="_wpnonce" value="465410aa16">
<input type="hidden" name="_wp_http_referer" value="/wordpress/wp-admin/admin.php?page=fahad_cpt">
<input type="submit" name="submit" id="submit" class="button btn btn-danger btn-sm" value="Delete">
<input type="hidden" name="option_page" value="fahad_plugin_cpt_settings">
<input type="hidden" name="action" value="update">
<input type="hidden" id="_wpnonce" name="_wpnonce" value="465410aa16">
<input type="hidden" name="_wp_http_referer" value="/wordpress/wp-admin/admin.php?page=fahad_cpt">
<input type="submit" name="submit" id="submit" class="button btn btn-danger btn-sm" value="Delete">

I know my there is a problem with my way of writing code but I couldn't figure it out after googling so need help with some concept explanation

Answer 1

You can use output buffering like:

$output_settings .= '';

ob_start();
settings_fields('fahad_plugin_cpt_settings');
submit_button('Delete', 'btn btn-danger btn-sm', 'submit', false);
$output_settings .= ob_get_clean();

and use $output_settings variable in your code like

// ...your code...
$data .= '<form method="post" action="options.php">';
$data .= $output_settings;
$data .= '</form>';
// ...your code...