How to return values in a function in php?

Assign the printable ones to a variable and then return them like this:

<?php

function sidebarContent($id) {
    switch ($id) {
        case "21":
        $return = '<p>Hello ABC.</p>
        <p class="content-register"><a href="/abc">Abc</a></p>';
        break;
        case "31":
        $return = '<p>Hello DEF.</p>
        <p class="content-register"><a href="/Def">Def</a></p>';
        break;
        case "41":
        $return = 'Your favorite color is green!";
        <p>Hello GHI.</p>
        <p class="content-register"><a href="/Ghi">Ghi</a></p>';
        break;
        default:
        $return = '<p>Hello World.</p>';
    }

    return $return;
}

echo sidebarContent(31);

Will prints:

Hello DEF.

Def

Demo

You don't have to store anything, you can indeed just output right away, unless you have further filter or handling steps to follow. Then indeed you can assign the strings to a variable by means of the = operator, or you can use "output buffering", or, as shown below, just directly return the literal markup.

If you decide to return the payload, then I suggest that approach, it allows clean html markup that is still readable:

<?php
function sidebarContent($id) {
    switch ($id) {
        case 21:
            return <<<EOT
<p>Hello ABC.</p>
<p class="content-register"><a href="/abc">Abc</a></p>
EOT;
        case 31:
            return <<<EOT
<p>Hello DEF.</p>
<p class="content-register"><a href="/Def">Def</a></p>  
EOT;
        case 41:
            return <<<EOT
Your favorite color is green!
<p>Hello GHI.</p>
<p class="content-register"><a href="/Ghi">Ghi</a></p>          
EOT;
        default:
            return <<<EOT
<p>Hello World.</p>
EOT;
    }
}

echo sidebarContent(111);

And note the missing closing php tag (?>). Do not use it. It creates huge issues and does not really help in most cases...

It's a better approuch to keep the data stored in a variable as simple as possible in order to have less memory used. As for your switch statement I can figure out that most of the cases use same format, that leads me to separate the specific date for each of them.

<?php
function sidebarContent($id) {
    $name = 'World';
    $extra = '';

    switch ($id) {
        case "21":
            $name = 'Abc';
            break;
        case "31":
            $name = 'Def';
            break;
        case "41":
            $name = 'Ghi';
            $extra = 'Your favorite color is green!';
    }

    return $extra . ' <p>Hello ' . strtoupper($name) . '.</p>' . 
           '<p class="content-register"><a href="/' . $name . '">' . $name . '</a></p>';
}

So the above code says that for each case amend the data stored in the 2 variables (at least 1 for each of them) and then return my HTML code and the data of the stored variables. This way you keep your code clean and simple.

A call to this function could be like:

echo sidebarContent(21); // Output the template
$my_sidebar = sidebarContent(21); // Store the template for later statements

Note: If you keep this function within a PHP file without HTML code, you can remove the closing PHP tag: ?>