If you use a regex, you're going to have to quotemeta one of the strings because '.' is a meta (regex command) character.

If you use substr, you'll have problems if the length of the part you are trying to match can vary. Ie. Could the strings be '13.2.3' & '13.2' for example.

Using index will allow you to avoid both these problems. You simple want to know if the shorter string matches the longer string at position 0:

$var1 = '1.1.2'; 
$var2 = '1.1'; 
if( index( $var1, $var2 ) == 0 ) { 
    print "matches" 
} else { 
    print "Doesn't match" 
}
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