The php function does not return the string and the line breaks

Two issues with the below php function.

One problem with the below function is the function echoes the $boo and $boo2 variables where the function is (i.e. before 1 and 2 is echoed). Both functions are echoed before 1 and 2. I'm unsure how to return the variables where each function is called without prematurely ending the function with 'return'.

The $boo variables are single line breaks or paragraphs of text.

<?php
function listformat ($list) {
    $listformat = explode("\n", $list);
    echo '<ul>';
    foreach ($listformat as $line) {
        echo "<li>".$line."</li>";
    };
    echo '</ul>';
};
listformat($boo);
echo '1';
listformat($boo2);
echo '2';
?>

Also wondering how I can remove line breaks of the $boo variables within the function when the lines of text have more than 1 line break.

EDIT: sample $boo is:

aaaa aaaaa

bbb bbbb

cccc c cc

Sometimes it can have more or less line breaks in between.

---

function listformat ($list) {
    $listformat = explode("\n", $list);
    $out = '<ul>';
    foreach ($listformat as $line) {
        $out .= "<li>".$line."</li>";
    };
    $out .= '</ul>';
    return $out;
};