"a1 b.d"<a1@test.com>,
<a2@test.com>,
"a3 b.d"<a3@test.com> ,
a4@test.com ,

a1@test.com,
a2@test.com,
a3@test.com,
a4@test.com,

function extractEmail( $data )
{
    if ( preg_match( "/[^@s]+@([-a-z0-9]+.)+[a-z]{2,}/i", $data, $email ) )
    {
        $replacement = '<a href="mailto:' . $email[ 0 ] . '" target="_blank">' . $email[ 0 ] . '</a> ';
        $data        = preg_replace( "/[^@s]+@([-a-z0-9]+.)+[a-z]{2,}/i", $replacement, $data );
    }
    return $data;
}

<?php
  echo extractEmail("Some text goes here before mail@mail.com and some text here goes after");
?>

function extract_emails_from($string) {
         preg_match_all("/[._a-zA-Z0-9-]+@[._a-zA-Z0-9-]+/i", $string, $matches);
         return $matches[0];
}