PHP函数的几大基本类型+引用的区别

$b = 10;
$b = &$a;
function test(&$a)
{
    $a = 30;
    echo $a;
}
test($b);
echo $b;//30

$a = 'abs';//函数
echo abs(-100);//100

function sum($a,$b)
{
    echo $a+$b;
}
function rel($a,$b,$fs='')
{
    return $fs($a,$b);//调用函数
}
$c = rel(10,20,'sum');
echo $c;

function sum1($a)
{
    if($a>0){

       return $a+sum1($a-1);
    }
    if($a==0){
        return $a=0;
    }
    if($a<0){
        return "请输入一个正数";
    }

}

$a = sum1(5);
echo "
";
echo $a;//15
echo "
";
$a = sum1(0);
echo "
";
echo $a;//0

$a = function($b)
{
    echo  $b;
};
echo $a(5)

$c = 10;
$a =function ($b) use(&$c)
{
    return $b+$c;
};
echo $a(5);