How to understand PHP substr() function usage: If the start parameter is a negative number and length is less than or equal to start, then length is 0. This sentence
If a negative length is provided, length characters at the end of the string will be omitted (if start is a negative number, it will be counted from the end of the string). If start is not in this text, FALSE will be returned. Reference: http://php.net/manual/zh/func...
Look at sbbstr('abcdefg', -4, -3), so the character with length 3 at the end of the character will be omitted. The actual intercepted string is abcd;sbbstr('abcdefg', -4, -3),所以该字符末尾长度为3的字符会被省略真正被截取的字符串为abcd; 同理sbbstr('abcdefg', -4, -5)，当length小于等于start时，包括下标为-4之后的字符都被省略了，所以截取的字符串也是空，所以说length为0Similarly to sbbstr('abcdefg', -4, -5), when length is less than or equal to start , including characters after the subscript -4 are omitted, so the intercepted string is also empty, so length is 0
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substr('abcdefg', -4, -2) and substr('abcdefg', -4, -6)
Look at
sbbstr('abcdefg', -4, -3), so the character with length3at the end of the character will be omitted. The actual intercepted string isabcd;sbbstr('abcdefg', -4, -3),所以该字符末尾长度为3的字符会被省略真正被截取的字符串为abcd;同理
sbbstr('abcdefg', -4, -5)，当length小于等于start时，包括下标为-4之后的字符都被省略了，所以截取的字符串也是空，所以说length为0Similarly tosbbstr('abcdefg', -4, -5), whenlengthis less than or equal tostart, including characters after the subscript-4are omitted, so the intercepted string is also empty, solengthis0