binary_gap.php

<?php
/*
  A binary gap within a positive integer N is any maximal
  sequence of consecutive zeros that is surrounded by ones
  at both ends in the binary representation of N.
  
  Find longest binary gap from given N integer
*/
function longBinaryGap(int $a) : int
{
    $bin = decbin($a);
    $maxGapLen = 0;
    
    $prevKey = null;
    for ($key = 0; $key < strlen($bin); $key++){
        $val = intval($bin[$key]);
        
        if (1 === $val) {
            if (!is_null($prevKey)) {
                $gapLen = $key - $prevKey - 1;
                
                if ($gapLen > $maxGapLen) {
                    $maxGapLen = $gapLen;
                }
            }
            
            $prevKey = $key;
        }
    }
    
    return $maxGapLen;
}

$arr = [9, 529, 20, 15, 32, 42, 147, 483, 647];

foreach($arr as $val) {
    print_r([
      'number' => $val,
      'binary_number' => decbin($val),
      'long_binary_gap' => longBinaryGap($val),
    ]);
}