trigger_error always reports the line and file that trigger_error was called on. Which isn't very useful.

eg:

main.php:
<?php
include('functions.php');
$x = 'test';
doFunction($x);
?>

functions.php:
<?php
function doFunction($var) {
if(is_numeric($var)) {
 /* do some stuff*/
} else {
 trigger_error('var must be numeric');
}
}
?>

will output "Notice: var must be numeric in functions.php on line 6"
whereas "Notice: var must be numeric in main.php on line 4" would be more useful

here's a function to do that:

<?php

function error($message, $level=E_USER_NOTICE) {
$caller = next(debug_backtrace());
trigger_error($message.' in <strong>'.$caller['function'].'</strong> called from <strong>'.$caller['file'].'</strong> on line <strong>'.$caller['line'].'</strong>'."\n<br />error handler", $level);
}
?>

So now in our example:

main.php:
<?php
include('functions.php');
$x = 'test';
doFunction($x);
?>

functions.php:
<?php
function doFunction($var) {
    if(is_numeric($var)) {
         /* do some stuff*/
    } else {
         error('var must be numeric');
    }
}

function error($message, $level=E_USER_NOTICE) {
    $caller = next(debug_backtrace());
    trigger_error($message.' in <strong>'.$caller['function'].'</strong> called from <strong>'.$caller['file'].'</strong> on line <strong>'.$caller['line'].'</strong>'."\n<br />error handler", $level);
}
?>

now outputs:

"Notice: var must be numeric in doFunction called from main.php on line 4"