php语法错误

<?php
function fun($num1='',$num2='',$zf=''){
if($num1 == '' and $num2 == '' and $zf == ''){
for($q=1;$q<=6;$q++){
echo"请至少输入一个值","<br>";
}
}else if($num2 ==null and $zf ==null){
for($x=$num1;$x<=9;$x++){
for($y=1;$y<=9;$y++);
}echo"<br>";
}else if($zf ==null){
for($x=$num1;$x<=$num2;$x++){
for($y=1;$y<=$x;$y++);
}echo"<br>";
}
}else if ($zf==$rsort){
for($x=$num2;$x>=$num1;$x--){
for($y=1;$y<=$x;$y++);
}echo"<br>";
}
}
fun(1,2);
?>
运行之后提示语法错误,else if($zf==null)这一行,怎么错的?求指导

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