Probá así:
Código PHP:
<script>
cad='ratapodridainmundapodrida'
s='podrida'
String.prototype.substr_count = function()
{
var search = arguments[0];
var count = 0;
var pos = this.indexOf(search);
while ( pos != -1 ) {
count++;
pos = this.indexOf(search,pos+1);
}
return count;
}
alert(cad.substr_count(s))
</script>