<?php
$text = 'Я очень проголодался, не против бы пойти в столовую.';
$words = [
'проголодался' => 'site1.com',
'пойти в столовую' => 'site2.com'
];
 
$patterns = array_map(function($v) {
  return '#(' . $v . '\b)#iu';
}, array_keys($words));
 
$replacements = array_map(function($v) {
  return '<a href="' . $v . '">\1</a>';
}, array_values($words));
 
$text = preg_replace($patterns, $replacements, $text);
 
echo $text;

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