php在div之前和逗号之后获取字符串的一部分

$str = "This is a <div>sample</div> string";
$beforeDiv = strstr($str, '<div>', true);
echo $beforeDiv;

This is a 

$str = "This is a <div>sample</div> string";
$divPosition = strpos($str, '<div>');
$beforeDiv = substr($str, 0, $divPosition);
echo $beforeDiv;

This is a 

$str = "This is a sample string, with a comma";
$afterComma = strstr($str, ',');
echo $afterComma;

, with a comma

$str = "This is a sample string, with a comma";
$commaPosition = strpos($str, ',');
$afterComma = substr($str, $commaPosition);
echo $afterComma;

, with a comma