Well I'm kinda new to the whole php thing but i might be able to help a wee bit and if it is no help sure i've lost nothin.
To get a sentence out I might try using strtok() with a delimiter of "."
$test = "hello, my name is Joe. I am 20";
$delimiter = ".";
$sentence = strtok($test, $delimiter);
for me this outputted "hello, my name is Joe"