从特定关键字或位置遍历php

I want to find the string from

$str = "Hello this is dummy.test. in this string  
any character, any special character can appear. it is not a fixed type 
text and predefined. this string will not read manually check by the 
Programmer.Sorry for the bad English.";

my algo is fixed because some kind of problem. First I want to find the position of test keyword then want to go back from test to dummy keyword to find position of dummy keyword .

I also searched and also use the internet but no any function found in PHP which can traverse back in this way.

I also test strrpos() but not relevant and required result. Any solution please.

Output needed : dummy.test

Algo : find first right (test) and then right to left (dummy) only. not first right (test) and then from starting to left (dummy) .

strrpos Would be the right way. If I understand you correctly, this should work, using the offset parameter:

$test = strrpos($str, 'test');
$dummy = strrpos($str, 'dummy', $test);

This will find the last "dummy" that occurs before the last "test".

First find the position of test using strpos(), cut the string up until test with substr(), and then use strpos() again to find the position of dummy in the substring:

$substring = substr($str, 0, strpos($str, 'test'));
$dummy = strpos($substring, 'dummy');
echo $dummy;

Output:

14

Demo!

UPDATE

As per the question edit, the following function should do what you want. It's probably not the best solution and is a bit ugly, but that should get you started:

function searchstr($str) 
{
    $pos = strpos($str, 'test') + strlen($str);
    $substring = substr($str, 0, $pos);
    $dummypos = strpos($substring, 'dummy', TRUE);

    $words = explode(' ', $str);
    $chars = -1; 
    foreach($words as $word)
    {
      $chars += strlen($word);
      if($chars >= $dummypos)
      {
         return $word;
      }   
    }   
    return ''; 
}   

echo searchStr($str);

Output:

dummy.test.

Demo!