word_occurance.php

<?php 

// list of words to find in string
$words = array('barack obama', 'chicago', 'nba');

// string to search
$sentence = "Barack Obama is from Chicago. Barack Obama's favorite food it pizza.";

$res = array(); // results array

// see how many times each word occurs in the sentence
foreach($words as $word){
  $res[$word] = substr_count(strtolower($sentence),$word);
}

// dump the result array to show results
echo var_dump($res);

?>