This is a sample tweet https://tr.co/sdfhnjn3

This is a sample tweet <a href="https://tr.co/sdfhnjn3">https://tr.co/sdfhnjn3</a>

$string = preg_replace('~(?<!href=['"])https?://[w/._-&?]*(?!</a>)(?=[^w/._-&])~gs', '<a href="$0">$0</a>', $string);`;

$message = preg_replace('$(s|^)(https?://[a-z0-9_./?=&-]+)(?![^<>]*>)$i', ' <a href="$2" target="_blank">$2</a> ', $message." ");

$message = preg_replace('$(s|^)(www.[a-z0-9_./?=&-]+)(?![^<>]*>)$i', '<a target="_blank" href="http://$2"  target="_blank">$2</a> ', $message." ");

$tweets = [
    'This is a sample tweet https://tr.co/sdfhnjn3',
    'This is a sample tweet <a href="https://tr.co/sdfhnjn3">https://tr.co/sdfhnjn3</a>',
    'This is a sample tweet https://example.com/sdfhnjn3',
    'This is a sample tweet <a href="https://example.com/sdfhnjn3">https://example.com/sdfhnjn3</a>'
];

var_export(preg_replace('~<a [^>]+>.*?</a>(*SKIP)(*FAIL)|https?://(?:www)?tr.co/S*~', '<a href="$0">$0</a>', $tweets));

array (
  0 => 'This is a sample tweet <a href="https://tr.co/sdfhnjn3">https://tr.co/sdfhnjn3</a>',
  1 => 'This is a sample tweet <a href="https://tr.co/sdfhnjn3">https://tr.co/sdfhnjn3</a>',
  2 => 'This is a sample tweet https://example.com/sdfhnjn3',
  3 => 'This is a sample tweet <a href="https://example.com/sdfhnjn3">https://example.com/sdfhnjn3</a>',
)