PHP中substr函数的作用是返回字符串的一部分，其语法为“substr(string,start,length)”，其参数string表示返回其中一部分的字符串，参数start表示在字符串的何处开始，参数length表示截取长度。

使用示例<?php

echo substr("Hello world",10)."
";

echo substr("Hello world",1)."
";

echo substr("Hello world",3)."
";

echo substr("Hello world",7)."
";

echo substr("Hello world",-1)."
";

echo substr("Hello world",-10)."
";

echo substr("Hello world",-8)."
";

echo substr("Hello world",-4)."
";

?><?php

echo substr("Hello world",0,10)."
";

echo substr("Hello world",1,8)."
";

echo substr("Hello world",0,5)."
";

echo substr("Hello world",6,6)."
";

echo substr("Hello world",0,-1)."
";

echo substr("Hello world",-10,-2)."
";

echo substr("Hello world",0,-6)."
";

echo substr("Hello world",-2-3)."
";

?>

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