PHP Project Swap

<?php

function trk_fastregex( $pagedata = '' )
{
    $pattern = '/([\w\s.]*)({{(prj)\|([^|{}.]*)}})([\w\s.]*)/';
    $replace = '{{$1}}<a href="./?({{$3}})={{$4}}">{{$4}}</a>{{$5}}';
    $subject = $pagedata;
    $matches = array();

    for( ; preg_match_all( $pattern, $pagedata, $matches ) ; )
    {
        foreach( $matches as $key => $value )
        {
            $subject =

                str_replace(

                '{{' . '$' . strval( $key + 1 ) . '}}',

                $value,

                $subject

                );
        }

        $matches = array();
    }

    $pagedata = $subject;

    return $pagedata;
}

?>

Error Line:
                $subject

Error Message:
Fatal error: Uncaught TypeError: str_replace(): Argument #2 ($replace) must be of type string when argument #1 ($search) is a string in C:\xampp8\htdocs\puzzlum.net\php\trk swaps.php:22 Stack trace: #0 C:\xampp8\htdocs\puzzlum.net\php\trk swaps.php(22): str_replace('{{$1}}', Array, '<style>{{font.m...') #1 C:\xampp8\htdocs\puzzlum.net\index.php(129): trk_fastregex('<style>{{font.m...') #2 {main} thrown in C:\xampp8\htdocs\puzzlum.net\php\trk swaps.php on line 22