For instance, if I had the string "X. Xavier Reynolds", and I wanted to easily replace just the first occurence of the X character, I would have to jump through hoops to split the string first to replace only the first occurrence.

Luckily, I found a great snippet function posted by xinobit on the PHP documentation web site. I include it in almost all of my PHP apps:

function str_replace_count($search,$replace,$subject,$times) {
    $subject_original=$subject;
    $len=strlen($search);    
    $pos=0;
    for ($i=1;$i<=$times;$i++) {
        $pos=strpos($subject,$search,$pos);
        if($pos!==false) {                
            $subject=substr($subject_original,0,$pos);
            $subject.=$replace;
            $subject.=substr($subject_original,$pos+$len);
            $subject_original=$subject;
        } else {
            break;
        }
    }
    return($subject);
}

?> 

To use it, just call the function, like this:

$myString = "X. Xavier Reynolds";
print str_replace_count("X","A",$myString,1);
?>