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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​
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<?php
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echo substr("Hello world",10)."<br>";
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echo substr("Hello world",1)."<br>";
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echo substr("Hello world",3)."<br>";
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echo substr("Hello world",7)."<br>";
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​
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echo substr("Hello world",-1)."<br>";
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echo substr("Hello world",-10)."<br>";
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echo substr("Hello world",-8)."<br>";
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echo substr("Hello world",-4)."<br>";
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?>
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</body>
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</html>
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​
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​
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​