One to find the closest to the value of the problem, please master to give a thought

One to find the closest value problem, please master to give a thought
To make a thing where the value of a is known, you need to find the value of b,b less than A and is 2 squared.
For example: a=765, ask for B, then B is equal to 512. I think of a way, because the value of a has the largest range, so I put 2 of 1, 2 of 2, 2 three. , save the array, and then use the a value and the array comparison to find the B value that meets the requirements.

But feel there should be other better ways, please master to give a thought. Thank you!

------Solution--------------------
function Get2str ($n)
{
$len = strlen (Decbin ($n));
$str 2 = ' 1 '. Str_repeat (' 0 ', $len-1);
Return Bindec ($str 2);
}
Var_dump (Get2str (578));
INT (512)
The idea is this:
4:100 3
8:1000 4
16:10,000 5
32:100,000 6
64:1 million 7
128:10 million 8
256:100 million 9
512:1 billion 10
1024:10 billion 11
2048:100 billion 12
578:1,001,000,010 10
As you can see, it is the decimal number: the binary string length
A binary string of 578 and 512 is 10 bytes long. I do not have PHP bit operations, so I think there must be a more efficient and simple solution.
------Solution--------------------
Please review the index and logarithm

PHP Code

$a = 765;echo pow (2, Floor (log ($a, 2)));
------Solution--------------------
1, leave the leftmost 1 on the line, bitwise operation.

  
  

   
   PHP Code
   
   
[Liangdong@bb-browser-test00.vm.baidu.com php_project]$ php main.php 765 = 5120 = 0256 = 2561 = 2 255 = 128257 = 256[liangdong@bb-browser-test00.vm.baidu.com php_project]$ cat main.php
    > $off)! = 1) {+ + $off;        } return 1 << $off; }} $cases = Array (765, 0, N, 2.3, 1, 3, 255, 257), foreach ($cases as $value) {if ($ret = Bit_alg ($value))!== FA LSE) {echo $value. "=". $ret.        Php_eol; }}?>
------Solution--------------------
If you calculate in binary mode, then
    
    

     
     

       PHP code 
      
 $A = 765; $t = Decbin ($A); Echo Bindec (Str_pad (1, strlen ( $t), ' 0 '); 
------Solution--------------------
 
        
         to discuss  
        
 
        
 If you are computing in binary mode, then PHP code 
        
 $A
   = 765;
   
        
 $t = Decbin ($A);
   
        
 Echo Bindec (Str_pad (1, strlen ($t), ' 0 ')); 
        
 
        
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