PHP - A Faster Way To Do "search And Replace"?

Hi everyone!! I just started learning PHP two weeks ago and I hope someone can help me!!
I need to do a find and replace on a string, and each replace needs to have an unique id, and if that unique id exists in a table then return the value on the table. Make sense?

So like "find all the Xs in the string, replace with "X1, X2, X3...", then if "X3' exists on TableB, return value "X3 is on the table and has a value of Y3".

I actually got this to work already but it takes a long time to do it so I'm hoping someone could give me some advice on how to make it more EFFICIENT:



// PART ONE, find all "X" and replace with unique_id

for ($count = 1; $count <= strlen($string); $count++) // do this loop as many times as there are characters in $string
{
$unique_id = "replacement_" . $count; // give each replacement an unique id (replacement_1, replacement_2, replacement_3, etc..)
$string = preg_replace("X", $unique_id, $string , 1); // find X and replace it with the $unique_id in $string, and only do this ONCE
} 
$count_replacements = substr_count($string,"replacement_"); // count how many replacements occured in total


// PART TWO, find all unique_ids in tableB and echo the unique_id_value

for ($count = 1; $count <= $count_replacements; $count++) // do this loop as many times as there were replacements
{
$unique_id  = "replacement_" . $count; // set the unique_id of each replacement


$show_tableB = mysqli_query($link, 'SELECT * FROM tableB');      // select  the unique_ids and the unique_id_values in TableB
while ($row = mysqli_fetch_array($show_tableB)) { $tableBs[] = array('unique_id' => $row['unique_id'], 'unique_id_value' => $row['unique_id_value']);  }    

foreach($tableBs as $tableB): // for each entry in TableB, if the unique_id in TableB matches the unique_id of the replacement, return the unique_id_value
if ($tableB['unique_id'] == $unique_id) 
{ echo $unique_id . " is on TableB and it has a value of ". $tableB['unique_id_value'] }
endforeach;

}



The problem with this is: its not very efficient.
If $string has 500 charachters, it does the "PART ONE" 500 times. Then, say it found 300 Xs, it does "PART TWO" 300 more times. And every time it does "PART TWO", it checks all entries in TableB (which could be 200 or more).

So, that's it. Anyone has a better idea of how to do this??

Thanks!!!!