I prefer to use regexps instead of your example due to brevity of handling the failure cases. Even so, you can tweak yours to be a bit more resilant.

This bit doesn't take into account not having 30 characters before the search string:

  $idx = index( $line, "Search String", 16 );
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Since we know there must be 30 bytes of data before the Search String, do this:

  $idx = index($line, "Search String", 16+30);
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The same idea is true for the "third_part" bit in your code and isn't hard to handle.