strpos() returns false if needle is not found in haystack. false is 0. Strings are indexed starting at 0. How should the caller of strpos() that strpos() didn't find the character at the beginning of the string. For example,...

printf( "pos is %d<br>\n", strpos( "abcdefg", "z" ) );
printf( "result is %s<br>\n", ( strpos( "abcdefg", "z" ) ) ? "true" : "false" );
printf( "pos is %d<br>\n", strpos( "abcdefg", "a" ) );
printf( "result is %s<br>\n", ( strpos( "abcdefg", "a" ) ) ? "true" : "false" );

...displays...

pos is 0
result is false
pos is 0
result is false

Most languages that have 0th indexed strings have strpos() return -1 when needle is not found in haystack. Either that, or the language has a special value for false (such as NULL instead of 0).