Run template inside a function from a file

<?php
include_once $modx->config['base_path'].'template.php';

$a='a variable';
template(); // I'm trying to display the result
?>

<?php
function template(){ // This was meant to be a display function
  global $a;
  echo 'I\'m calling '.$a;
}
?>

<?php
global $a;

function template(){ // This was meant to be a display function
  global $a;
  return 'I\'m calling '.$a;
}

$a = 'a variable';
return template($a);
?>

<?php
function template($a){ // This was meant to be a display function
  return 'I\'m calling '.$a;
}

$a = 'a variable';
return template($a);
?>

<?php
$a = 'a variable';
return template($a);

<?php
$a = 'a variable';
echo template($a);

file: snippet.inc.php
<?php
global $a; // different with a standalone php file
$a = 'a variable';
return template();
?>

file: template.php
<?php
function template(){ // This was meant to be a display function
  global $a;
  echo 'I\'m calling '.$a;
}

file: snippet.inc.php
<?php
global $a; // different with a standalone php file
$a = 'a variable';
echo template(); // I think this one, since the "echo" doesn't immediately end execution the rest of the codes.
?>

file: template.php
<?php
function template(){ // This was meant to be a display function
  global $a;
  return 'I\'m calling '.$a;
}