WordPress snippet to replace first occurrence of a string with another within post content, relying on the_content filter and the PHP preg_replace function. In this example, the first occurence of "WordPress" is being replaced with a link to wordpress.org. The last parameter on line 4 restricts the replacement to only the first occurrence.

replace_first_occurrence.php

<?php
function replace_content($content)
{
    $content = preg_replace('/WordPress/', '<a href="http://wordpress.org/">WordPress</a>', $content, 1);
    return $content;
}
add_filter('the_content','replace_content');
?>