$c = [];
foreach ( $t as $p ) {
      $c[] = $p->id;
}
$c = join( ', ', $c );

<?php

    $data           = 'my secrect string...';
    $key            = md5( 'passwd123' );

    $crypted        = $data ^ str_pad( '', strlen( $data ), $key );
    print_r( $crypted ); // \HGZPLCE_VTJ

    $decrypted      = $crypted ^ str_pad( '', strlen( $crypted ), $key );
    print_r( $decrypted ); // my secrect string...

    // меняем пасс
    $decrypted      = $crypted ^ str_pad( '', strlen( $crypted ), md5( 'passwd321' ) );
    print_r( $decrypted ); // m{r"6<"ae{zw+poc0u)