PHP实现判断是否是连乘数字串的方法

/**
 * 判断是否连乘数字串函数
* @param $strNum
 * @return bool
 **/
function isExec($strNum) {
 $str = (string)$strNum;

 for ($i = 0; $i < strlen($str); $i++) {
  $k = 1;

  for ($j = $i + 1; $j < strlen($str); $j++) {

   $a_i = 0;
   $b_i = $i + 1;
   $p_i = $i + 1;
   $m_i = $k++;
   $c_i = $b_i + $m_i;
   $res = false;
   $formula = [];

   while (1) {
    $a = substr($str, $a_i, $p_i);
    $b = substr($str, $b_i, $m_i);

    $n = $a * $b;

    $c = substr($str, $c_i, strlen($n));

    //echo &#39;<br/>&#39; . $a . &#39;*&#39; . $b . &#39;=&#39; . $n . &#39;->&#39; . $c . &#39;<br/>&#39;;

    if($c){
     $formula[] = $a . &#39;*&#39; . $b . &#39;=&#39; . $n;
    }

    if ($c === false || $c === "") {
     break;
    }

    if (intval($n) == intval($c)) {
     $p_i = strlen($b);
     $m_i = strlen($n);
     $a_i = $b_i;
     $b_i = $c_i;
     $c_i = $b_i + $m_i;
     $res = true;
    } else {
     $res = false;
     break;
    }
   }

   if ($res === true) {
    print_r($formula);
    return true;
   }

   //var_dump($res) . &#39;<br/>&#39;;
  }

 }

 return false;
}

//运行结果
Array
[
   [0] => 1*2=2
   [1] => 2*2=4
   [2] => 2*4=8
   [3] => 4*8=32
   [4] => 8*32=256
]
bool(true)

//运行结果
Array
[
   [0] => 23454*7898=185239692
]
bool(true)

//运行结果
Array
[
   [0] => 11*22=242
]
bool(true)

//运行结果
bool(false)