Casual php user.
I can't find out how to accomplish this.
I have an associative array, some elements have a value, some don't but all have a "string key". When an element is found to have a value, the key and the value must be exchanged leading to a subsequent sort for presentation.
For example in a foreach loop
foreach ($array as $key => $value)
$key must be assigned the content of $value and $value must be assigned the content of $key if both $key and $value have a content.

Your help is much appreciated.

well you could do inside the loop if $value > '' .. or if $value != null
$array[$value] = $key; // This creates a new element ... and then you should extract the the $key key from the array and rebuild it ... Probably php has a function for this .. but it's also trivial to implement

Thank you for your answer.
I have tried a number of variations of your solution without success. The PHP documentation gives the following example:

<?php
$arr = array(1, 2, 3, 4);
foreach ($arr as &$value) {
$value = $value * 2;
}
// $arr is now array(2, 4, 6, 8)
?>

but the comment is wrong, the element in the array is unchanged after this foreach loop and what I need to know is how to change it. It may be trivial but I have no idea how to do this.

There is a function to do this on a whole array but no function to do it on individual elements of an array.

For anyone interested, I found the only way is to create a new array as described below and then use this new array for sorting and display.
The original array was $fils

foreach ($fils as $path => $name) {
if ($name) {
$fils2[$name]=$path;
}
else {
$fils2[$path]="";
}
}