PHP引用函数不在脚本中

Can somebody please explain to me where can PHP script get this functions from, I have a PHP script that has this few lines:

<?php
$args1 = array();
$gethosts = get_xml_host_objects($args1); //grabbing internal xml data from backend

$args2 = array();
$gethoststatus = get_xml_host_status($args2);

$args3 = array();
$getparenthosts = get_xml_host_parents($args3);

So I do not understand from where and how could this PHP script be referencing those functions, could somebody just give me few examples suggestions of where to look?

Maybe, there is a file that includes the two scripts. Somethings like that

<?php
    include 'file_with_function.php';
    include 'your_file.php';
?>

These lines can help. Place them at the beginning of your file

echo '<pre>';  
debug_print_backtrace();  
echo '</pre>';

It looks like those were self defined functions, maybe you can try grep thru entire source tree to see if which .php file define these functions and include it in this script.