how do i remove the matched character from a string?

here is my code

$a = "Hey there, how do i remove all, the comma from this string,";
$a = str_replace($a,',','';)
echo $a;

i want to remove all the commas available in the string, 开发者_如何学Gohow do i do it?

$a = str_replace(",", "", $a);

$a = "Hey there, how do i remove all, the comma from this string,";
$a = str_replace(',','',$a);
echo $a;

Misplaced semi-colon and wrong function arguments.