Counting characters in a thread

[Jimstone77]

$string = 
"a%3A2%3A%7Bi%3A0%3Bs%3A3%3A%22489%22%3Bi%3A1%3Bs%3A32%3A%22a85a44c1881523798bc155a5369e1226%22%3B%7D"

How would I empty $string if it contained more than ten % characters? In 
other words

$string = 
"a%3A2%3A%7Bi%3A0%3Bs%3A3%3A%22489%22%3Bi%3A1%3Bs%3A32%3A%22a85a44c1881523798bc155a5369e1226%22%3B%7D"

Would be emptied, while

$string = "a%3A2%3A%7Bi%3A0%3Bs%3A3%3A%22489%22"

would not.