php strpos offset输出不同

Question

What is it that causes this difference?

echo strpos('abcdef abcdef', 'a', 0); //output= 0
echo strpos('abcdef abcdef', 'a', 1); //output= 7
echo strpos('a b- c a b* c a b+ c', 'c', 0); //output= 5 
echo strpos('a b- c a b* c a b+ c', 'c', 1); //output= 5 (But why? Is is must be 12)
echo strpos('a b- c a b* c a b+ c', 'c', 6); //output= 12

Answer 1

The 3rd argument is the starting location, not the number of found characters to skip. e.g.

strpos('...', 'a', 0)   option #1
strpos('...', 'a', 1)   option #2

abcdef abcdef
0123456789111
↑↑        012
││
└┼─ option #1: start at position 0('a'), and boom, find an 'a'
 └─ option #2; start at position 1('b'), find 'a' at position 7

So for your a b- c a b* c a b+ c string, the second version only skips 1 character, leaving the first c to be found at position 5:

a b- c a b* c a b+ c
01234567891111111111
↑↑    ↑   0123456789
││    │
└┼────┼─ start at position 0, first C at position 5
 └────┼─ start at position 1, first C still at 5
      └─ start at position 6, first C now at 12