Can't access global variable inside function

You have to pass it to the function:

<?php
    $sxml = new SimpleXMLElement('<somexml/>');

    function foo($sxml){
        $child = $sxml->addChild('child');
    }

    foo($sxml);
?>

or declare it global:

<?php
    $sxml = new SimpleXMLElement('<somexml/>');

    function foo(){
        global $sxml;
        $child = $sxml->addChild('child');
    }

    foo();
?>

If the variable isn't global but is instead defined in an outer function, the first option (passing as an argument) works just the same:

<?php
    function bar() {
        $sxml = new SimpleXMLElement('<somexml/>');
        function foo($sxml) {
            $child = $sxml->addChild('child');
        }
        foo($sxml);
    }
    bar();
?>

Alternatively, create a closure by declaring the variable in a use clause.

<?php
    function bar() {
        $sxml = new SimpleXMLElement('<somexml/>');
        function foo() use(&$xml) {
            $child = $sxml->addChild('child');
        }
        foo();
    }
    bar();
?>

You need to explicitly invite the global variable into the functions scope:

function foo(){
    global $sxml;
    $child = $sxml->addChild('child');
}

Use the global keyword to declare $sxml inside your function.

<?php
    $sxml = new SimpleXMLElement('<somexml/>');
    function foo(){
    global   $sxml;  
    $child = $sxml->addChild('child');
    }
    foo();
?>

another solution is to use $GLOBALS while you declare that variable:

         $my_var   = 'blabla';    // not global
$GLOBALS['my_var'] = 'blabla';    // global  (correct)

While the top answer provides a nice solution, I'd like to argue that the appropriate solution in most modern PHP applications is to create a class with a static variable, like so:

<?php

class xmlHelper {
    private static $sxml;

    public function getXML() {
        return self::$sxml;
    }

    public function setXML($xml) {
        self::$sxml = $xml;
    }
}

xmlHelper::setXML(new SimpleXMLElement('<somexml/>'));

function foo(){
    $child = xmlHelper::getXML()->addChild('child');
}

foo();

This approach allows you to access $sxml from within foo() just like you wanted, but it has a few advantages over the global approach.

1. With this strategy, you will always be able to put a breakpoint inside setXML() to find out what part of your application has manipulated this value, which you cannot do when manipulating globals.
2. You avoid polluting the global namespace with a generic variable name sxml.