Trying to setup a function

Do you want to return something from the function?  Nothing is returned as of yet.  Functions have non-shared variable scopes by default so you would need to use the global function to use a var outside of a function that you set to a value within the function.  Also, do you want to create a variable variable name such as ${$field} or did you just want a var named field?

//Set Function
$field='';
function check_field($field) {
  global $field;
		if (isset($_SESSION[$field])) {
		$field = $_SESSION[$field];
		} else {
		$field='';
		}
 
//Call function	
check_field('M_buss_name');

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Still had some syntax issues, should read:

<?php
$field='';
$_SESSION['M_buss_name']="some value";
function check_field($fld) {
  global $field;
  $field = isset($_SESSION[$fld]) ? $_SESSION[$fld] : '' ;
}
check_field('M_buss_name');
echo $field;
?>

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I wanted a function that would do what I'm doing below, later in the page I print the variable either blank or with whatever is in it.

Per the code below, it doesn't simplify me writing below over and over again because I'll need to define:
$_SESSION['M_buss_name']="some value";

Will I need that there?

if ( isset($_SESSION['MC_vatnr'])) {
	$MC_vatnr=$_SESSION['MC_vatnr'];
	} else {
	$MC_vatnr='';
	}

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Beautiful, thank you!