string number_format ( float$number [, int$decimals = 0 ] )
string number_format ( float$number , int$decimals = 0 , string $dec_point = '.' , string $thousands_sep = ',' )

function num_format($num){
 if(!is_numeric($num)){
 return false;
 }
 $num = explode('.',$num);//把整數(shù)和小數(shù)分開
 $rl = $num[1];//小數(shù)部分的值
 $j = strlen($num[0]) % 3;//整數(shù)有多少位
 $sl = substr($num[0], 0, $j);//前面不滿三位的數(shù)取出來
 $sr = substr($num[0], $j);//后面的滿三位的數(shù)取出來
 $i = 0;
 while($i <= strlen($sr)){
 $rvalue = $rvalue.','.substr($sr, $i, 3);//三位三位取出再合并，按逗號隔開
 $i = $i + 3;
 }
 $rvalue = $sl.$rvalue;
 $rvalue = substr($rvalue,0,strlen($rvalue)-1);//去掉最后一個逗號
 $rvalue = explode(',',$rvalue);//分解成數(shù)組
 if($rvalue[0]==0){
 array_shift($rvalue);//如果第一個元素為0，刪除第一個元素
 }
 $rv = $rvalue[0];//前面不滿三位的數(shù)
 for($i = 1; $i < count($rvalue); $i++){
 $rv = $rv.','.$rvalue[$i];
 }
 if(!empty($rl)){
 $rvalue = $rv.'.'.$rl;//小數(shù)不為空，整數(shù)和小數(shù)合并
 }else{
 $rvalue = $rv;//小數(shù)為空，只有整數(shù)
 }
 return $rvalue;
}

echo number_format('169856420');

echo number_format('1000000',2);

echo number_format('1000000',2,',','.');