1) If the two strings have identical BEGINNING parts, they are trunkated from both strings.
2) The resulting strings are compared with two possible outcomes:
a) if one of the resulting strings is an empty string, then the length of the non-empty string is returned (the sign depending on the order in which you pass the arguments to the function)
b) in any other case just the numerical values of the FIRST characters are compared. The result is +1 or -1 no matter how big is the difference between the numerical values.
<?php
$str = array('','a','afox','foxa');
$size = count($str);
echo '<pre>';
for($i=0; $i<$size; $i++)
{
for($j=$i+1; $j<$size; $j++)
{
echo '<br>('.$str[$i].','.$str[$j].') = '.strcmp($str[$i], $str[$j]);
echo '<br>('.$str[$j].','.$str[$i] .') = '.strcmp($str[$j], $str[$i]);
}
}
echo '</pre>';
?>
In Apache/2.4.37 (Win32) OpenSSL/1.1.1 PHP/7.2.12 produces the following results:
(,a) = -1 //comparing with an empty string produces the length of the NON-empty string
(a,) = 1 // ditto
(,afox) = -4 // ditto
(afox,) = 4 // ditto
(,foxa) = -4 // ditto
(foxa,) = 4 // ditto
(a,afox) = -3 // The identical BEGINNING part ("a") is trunkated from both strings. Then the remaining "fox" is compared to the remaing empty string in the other argument. Produces the length of the NON-empty string. Same as in all the above examples.
(afox,a) = 3 // ditto
(a,foxa) = -1 // Nothing to trunkate. Just the numerical values of the first letters are compared
(foxa,a) = 1 // ditto
(afox,foxa) = -1 // ditto
(foxa,afox) = 1 // ditto
