<?php

$str = 'Example: hello world, this world rocks.
What it should do is: 
if it finds the word hello it should
remove the whole line. How can i do that and there 
could be words in between brackets and inverted commas also.';

$lines = explode("\n", $str);

foreach($lines as $index => $line) {

   if (strstr($line, 'hello')) {
      unset($lines[$index]);
   }

}

$str = implode("\n", $lines);

var_dump($str);

string(137) "What it should do is: 
remove the whole line. How can i do that and there 
could be words in between brackets and inverted commas also."