The end() functions takes a reference to the variable as it's parameter, this means that it works with the actual variable instead of just the value. If we made a function like this
function double_value(&$value){
$value *= 2;
}
And used it like this
$number = 10;
double_value($number);
// HERE
Where I have commented //HERE the variable $number will contain the value 20 if we did the same thing with this function
function double_value($value){
$value *= 2;
}
it would still be 10 since the value is only modified inside the function.
The error is php complaining because you did not give it a variable for that function, meaning it cannot use the reference (since there is none). It does not matter though since it falls back to the value and works fine so you can just disable these messages.
error_reporting(E_ALL & ~E_STRICT);
or in your php.ini file
error_reporting = E_ALL & ~E_STRICT