You don’t display anything in shorcode’s function, you just return what has to be displayed. Your code should be:
function phpTest() {
return '<p>some text</p>';
}
add_shortcode('test', 'phpTest');
Update: if your function contains anything, which displays data, use it as follows:
function phpTest() {
ob_start();
// your code goes here
return ob_get_clean();
}
add_shortcode('test', 'phpTest');