案例:php案例     状态:不可编辑再运行    进入竖版
 运行结果 
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<!DOCTYPE html>
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<html>
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<body>
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<?php
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echo substr("Hello world",0,10)."<br>";
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echo substr("Hello world",1,8)."<br>";
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echo substr("Hello world",0,5)."<br>";
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echo substr("Hello world",6,6)."<br>";
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echo substr("Hello world",0,-1)."<br>";
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echo substr("Hello world",-10,-2)."<br>";
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echo substr("Hello world",0,-6)."<br>";
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echo substr("Hello world",-2-3)."<br>";
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?>
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</body>
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</html>
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