php - Syntax error, unexpected T_LNUMBER, expecting T_VARIABLE or '$'

I'm getting Syntax error, unexpected T_LNUMBER, expecting T_VARIABLE or '$'

This is the code i'm using

function wpse44503_filter_content( $content ) {
    $regex = '#src=("|\')'.
        '(/images/(19|20)(0-9){2}/(0|1)(0-9)/[^.]+\.(jpg|png|gif|bmp|jpeg))'.

        '("|\')#';
    $replace = 'src="'.get_site_url( $2 ).'"';

    $output = preg_replace( $regex, $replace, $content );

    return $output;
}

This is the line where i'm getting that error $replace = 'src="'.get_site_url( $2 ).'"';

Can anyone help me to fix it?
Thanks

Answer

What you're trying to do (ie replacing the matched string with the result of a function call) can't be done using preg_replace, you'll need to use preg_replace_callback instead to get a function called for every match.

A short example of preg_replace_callback;

$get_site_url =                    // Returns replacement
  function($row) { 

    return '!'.$row[1].'!';        // row[1] is first "backref"
  };                                                     

$str = 'olle';
$regex = '/(ll)/';                 // String to match

$output = preg_replace_callback(   // Match, calling get_site_url for replacement
    $regex,
    $get_site_url,
    $str);


var_dump($output);                 // output "o!ll!e"