Why is my WordPress plugin showing 'The plugin generated X characters of unexpected output during activation'?

The plugin generated X characters of unexpected output during activation (WordPress)

When activating a WordPress plugin, you may encounter the error message: "The plugin generated X characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin." This error can be frustrating, but understanding its cause and finding an effective solution is crucial.

Possible Reasons for the Error:

1. Incorrect Output Placement: Unexpected output refers to any content sent to the browser outside the designated WordPress hooks. This can occur if you output messages (e.g., echo) directly in the plugin's PHP code, which should be avoided during activation.
2. Unknown PHP Error: If you're not intentionally outputting any data, there might be an underlying PHP error. To identify it, you can temporarily add the following code to your functions.php file and activate the plugin. The error message will be displayed.

define('temp_file', ABSPATH.'/_temp_out.txt' );

add_action("activated_plugin", "activation_handler1");
function activation_handler1(){
    $cont = ob_get_contents();
    if(!empty($cont)) file_put_contents(temp_file, $cont );
}

add_action( "pre_current_active_plugins", "pre_output1" );
function pre_output1($action){
    if(is_admin() && file_exists(temp_file))
    {
        $cont= file_get_contents(temp_file);
        if(!empty($cont))
        {
            echo '<div class=&quot;error&quot;> Error Message:' . $cont . '</div>';
            @unlink(temp_file);
        }
    }
}

Effective Solutions:

1. Wrap Output in Conditional Statements: If your plugin requires outputting data, wrap it within an if statement that only executes when specific conditions are met. For example, check if the current page is not the plugins.php administration page.
2. Use Appropriate WordPress Hooks: Follow WordPress best practices and output messages using the correct hooks. For example, use admin_notices to display messages in the admin dashboard or the_content to display content on the front-end.
3. Eliminate Unknown PHP Errors: If you encounter an unknown PHP error, debug your plugin code and resolve the underlying issue. Referencing WordPress documentation and asking for community support can be helpful in troubleshooting errors.

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