null array mysqli_fetch_assoc ( mysqli_result $result ) void mysqli_free_result ( mysqli_result $result )

w取最大db_id。 

if($row=null){
    var_dump(123);
}
var_dump($row);

if($rowb=array('ww','w123')){
    var_dump(234);
}
var_dump($rowb);

$rowc=null;
if($rowc=null){
    var_dump(345);
}
var_dump($rowc);

	NULL
	int(234)
	array(2) {
	[0]=>
	string(2) "ww"
	[1]=>
	string(4) "w123"
	}
	NULL

Returns an associative array that corresponds to the fetched row or NULL if there are no more rows.

id  wint    wchar
1    1001    1001www
2    1002    1002www
3    1003    1003www
4    1004    1004www
5    1005    1005www
6    1006    1006www
7    1007    1007www
8    1008    1008www
9    1009    1009www
10    1010    1010www
11    1011    1011www
12    1012    1012www
13    1013    1013www
14    1014    1014www
15    1015    1015www
16    1016    1016www
UPDATE w1 SET wint=id+1000;
UPDATE w1 SET wchar=CONCAT(wint,'www');
DELETE FROM w1 WHERE id>16;

$sql = 'SELECT * FROM w1 ';
if ($result = mysqli_query($link, $sql)) {
    while ($row = mysqli_fetch_assoc($result)) {
        $w_id = $row['id'];
    }
   // mysqli_free_result($result);
}
var_dump($w_id);
var_dump($row);
var_dump($result);

string(2) "16" NULL object(mysqli_result)#3 (5) { ["current_field"]=> int(0) ["field_count"]=> int(3) ["lengths"]=> NULL ["num_rows"]=> int(16) ["type"]=> int(0) }

$sql = 'SELECT * FROM w1 ';
if ($result = mysqli_query($link, $sql)) {
    while ($row = mysqli_fetch_assoc($result)) {
        $w_id = $row['id'];
    }
    mysqli_free_result($result);
}
var_dump($w_id);
var_dump($row);
var_dump($result);

string(2) "16" NULL
Warning: var_dump() [function.var-dump]: Couldn't fetch mysqli_result
Warning: var_dump() [function.var-dump]: Couldn't fetch mysqli_result
Warning: var_dump() [function.var-dump]: Property access is not allowed yet
Warning: var_dump() [function.var-dump]: Couldn't fetch mysqli_result
Warning: var_dump() [function.var-dump]: Property access is not allowed yet
object(mysqli_result)#3 (5) { ["current_field"]=> NULL ["field_count"]=> NULL ["lengths"]=> NULL ["num_rows"]=> NULL ["type"]=> NULL }