能用PHP的octdec和decoct函數實現八進制與十進制的雙向轉換嗎？

gitbox 2025-09-12

—— 和正文無關的內容分隔線——

結論先說：可以。 octdec()用於把八進製字符串→ 十進制整數， decoct()用於把十進制整數→ 八進製字符串。兩者配合即可實現雙向轉換。下面把用法、易錯點與進階方案一次講清。

1. 基本用法

<span><span><span class="hljs-meta">&lt;?php</span></span><span>
</span><span><span class="hljs-comment">// 八進制 → 十進制</span></span><span>
</span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-title function_ invoke__">octdec</span></span><span>(</span><span><span class="hljs-string">'17'</span></span><span>);   </span><span><span class="hljs-comment">// 15</span></span><span>
</span><span><span class="hljs-keyword">echo</span></span><span> PHP_EOL;
</span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-title function_ invoke__">octdec</span></span><span>(</span><span><span class="hljs-string">'011'</span></span><span>);  </span><span><span class="hljs-comment">// 9（字符串裡的前導 0 不會改變含義，它仍按八進制解析）</span></span><span>

</span><span><span class="hljs-comment">// 十進制 → 八進制</span></span><span>
</span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-title function_ invoke__">decoct</span></span><span>(</span><span><span class="hljs-number">15</span></span><span>);     </span><span><span class="hljs-comment">// "17"</span></span><span>
</span><span><span class="hljs-keyword">echo</span></span><span> PHP_EOL;
</span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-title function_ invoke__">decoct</span></span><span>(</span><span><span class="hljs-number">9</span></span><span>);      </span><span><span class="hljs-comment">// "11"</span></span><span>
</span></span>

要點：

octdec(string $octal): int ——參數是字符串，返回十進制整數。
decoct(int $decimal): string ——參數是整數，返回八進制的字符串。

2. 常見陷阱與邊界

(1) 非法字符

八進制只允許0–7 。傳入如8、9或非數字字符會被忽略到第一個非法處之後，結果容易與你的預期不符。先校驗再轉換最穩妥。

 <span><span><span class="hljs-meta">&lt;?php</span></span><span>
</span><span><span class="hljs-function"><span class="hljs-keyword">function</span></span></span><span> </span><span><span class="hljs-title">isValidOctal</span></span><span>(</span><span><span class="hljs-params"><span class="hljs-keyword">string</span></span></span><span> </span><span><span class="hljs-variable">$s</span></span><span>): </span><span><span class="hljs-title">bool</span></span><span> {
    </span><span><span class="hljs-comment">// 允許前導 0；至少一个八進制数字</span></span><span>
    </span><span><span class="hljs-keyword">return</span></span><span> (</span><span><span class="hljs-keyword">bool</span></span><span>)</span><span><span class="hljs-title function_ invoke__">preg_match</span></span><span>(</span><span><span class="hljs-string">'/^[0-7]+$/'</span></span><span>, </span><span><span class="hljs-variable">$s</span></span><span>);
}

</span><span><span class="hljs-variable">$input</span></span><span> = </span><span><span class="hljs-string">'128'</span></span><span>; </span><span><span class="hljs-comment">// 非法</span></span><span>
</span><span><span class="hljs-keyword">if</span></span><span> (!</span><span><span class="hljs-title function_ invoke__">isValidOctal</span></span><span>(</span><span><span class="hljs-variable">$input</span></span><span>)) {
    </span><span><span class="hljs-keyword">throw</span></span><span> </span><span><span class="hljs-keyword">new</span></span><span> </span><span><span class="hljs-built_in">InvalidArgumentException</span></span><span>(</span><span><span class="hljs-string">"不是合法的八進制数：<span class="hljs-subst">{$input}</span></span></span><span>");
}
</span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-title function_ invoke__">octdec</span></span><span>(</span><span><span class="hljs-variable">$input</span></span><span>);
</span></span>

(2) 前導0 與代碼裡的“八進製字面量”

字符串"011"只是文本， octdec("011")會按八進制解析為9。
PHP 代碼裡的整數字面量若寫成011 ，這是八進製字面量（等於十進制9）。建議避免在源碼中使用前導0 的整數字面量，以免閱讀混淆；需要八進制時更清晰的做法是使用字符串配合octdec() 。

(3) 負數與位寬

decoct()的參數是整數。對負數時，它會按實現平台的位寬（通常64 位）以二進制補碼形式參與運算並給出一個看起來“很長”的八進製表示，這通常不是你要的結果。如無特別需求，請只處理非負整數。

 <span><span><span class="hljs-meta">&lt;?php</span></span><span>
</span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-title function_ invoke__">decoct</span></span><span>(-</span><span><span class="hljs-number">10</span></span><span>); </span><span><span class="hljs-comment">// 依平台而異，通常是很長的一串</span></span><span>
</span></span>

(4) 大整數

在64 位PHP 上， int最大約為9.22e18。 decoct()受此限制。
若需要處理超大十進制數（超過int範圍）的轉換，考慮：
- 使用gmp_* （如gmp_init + gmp_strval($n, 8) ）；
- 或使用bc /自寫大數算法。
八進制→十進制時， octdec()返回int ，同樣受限；超出時請改用gmp / bc方案。

3. 實用封裝：穩健的雙向轉換

八進製字符串→ 十進制（返回int ）

 <span><span><span class="hljs-meta">&lt;?php</span></span><span>
</span><span><span class="hljs-function"><span class="hljs-keyword">function</span></span></span><span> </span><span><span class="hljs-title">octalToDecimal</span></span><span>(</span><span><span class="hljs-params"><span class="hljs-keyword">string</span></span></span><span> </span><span><span class="hljs-variable">$oct</span></span><span>): </span><span><span class="hljs-title">int</span></span><span> {
    </span><span><span class="hljs-keyword">if</span></span><span> (!</span><span><span class="hljs-title function_ invoke__">preg_match</span></span><span>(</span><span><span class="hljs-string">'/^[0-7]+$/'</span></span><span>, </span><span><span class="hljs-variable">$oct</span></span><span>)) {
        </span><span><span class="hljs-keyword">throw</span></span><span> </span><span><span class="hljs-keyword">new</span></span><span> </span><span><span class="hljs-built_in">InvalidArgumentException</span></span><span>(</span><span><span class="hljs-string">"非法八進制：<span class="hljs-subst">$oct</span></span></span><span>");
    }
    </span><span><span class="hljs-keyword">return</span></span><span> </span><span><span class="hljs-title function_ invoke__">octdec</span></span><span>(</span><span><span class="hljs-variable">$oct</span></span><span>);
}
</span></span>

十進制（非負int ）→ 八進製字符串

<span><span><span class="hljs-meta">&lt;?php</span></span><span>
</span><span><span class="hljs-function"><span class="hljs-keyword">function</span></span></span><span> </span><span><span class="hljs-title">decimalToOctal</span></span><span>(</span><span><span class="hljs-params"><span class="hljs-keyword">int</span></span></span><span> </span><span><span class="hljs-variable">$dec</span></span><span>): </span><span><span class="hljs-title">string</span></span><span> {
    </span><span><span class="hljs-keyword">if</span></span><span> (</span><span><span class="hljs-variable">$dec</span></span><span> &lt; </span><span><span class="hljs-number">0</span></span><span>) {
        </span><span><span class="hljs-keyword">throw</span></span><span> </span><span><span class="hljs-keyword">new</span></span><span> </span><span><span class="hljs-built_in">InvalidArgumentException</span></span><span>(</span><span><span class="hljs-string">"只支持非負整數"</span></span><span>);
    }
    </span><span><span class="hljs-keyword">return</span></span><span> </span><span><span class="hljs-title function_ invoke__">decoct</span></span><span>(</span><span><span class="hljs-variable">$dec</span></span><span>);
}
</span></span>

進階：用GMP 支持超大數

<span><span><span class="hljs-meta">&lt;?php</span></span><span>
</span><span><span class="hljs-function"><span class="hljs-keyword">function</span></span></span><span> </span><span><span class="hljs-title">octalToDecimalBig</span></span><span>(</span><span><span class="hljs-params"><span class="hljs-keyword">string</span></span></span><span> </span><span><span class="hljs-variable">$oct</span></span><span>): </span><span><span class="hljs-title">string</span></span><span> {
    </span><span><span class="hljs-keyword">if</span></span><span> (!</span><span><span class="hljs-title function_ invoke__">preg_match</span></span><span>(</span><span><span class="hljs-string">'/^[0-7]+$/'</span></span><span>, </span><span><span class="hljs-variable">$oct</span></span><span>)) {
        </span><span><span class="hljs-keyword">throw</span></span><span> </span><span><span class="hljs-keyword">new</span></span><span> </span><span><span class="hljs-built_in">InvalidArgumentException</span></span><span>(</span><span><span class="hljs-string">"非法八進制：<span class="hljs-subst">$oct</span></span></span><span>");
    }
    </span><span><span class="hljs-variable">$g</span></span><span> = </span><span><span class="hljs-title function_ invoke__">gmp_init</span></span><span>(</span><span><span class="hljs-variable">$oct</span></span><span>, </span><span><span class="hljs-number">8</span></span><span>);
    </span><span><span class="hljs-keyword">return</span></span><span> </span><span><span class="hljs-title function_ invoke__">gmp_strval</span></span><span>(</span><span><span class="hljs-variable">$g</span></span><span>, </span><span><span class="hljs-number">10</span></span><span>); </span><span><span class="hljs-comment">// 返回十進制“字符串”</span></span><span>
}

</span><span><span class="hljs-function"><span class="hljs-keyword">function</span></span></span><span> </span><span><span class="hljs-title">decimalToOctalBig</span></span><span>(</span><span><span class="hljs-params"><span class="hljs-keyword">string</span></span></span><span> </span><span><span class="hljs-variable">$dec</span></span><span>): </span><span><span class="hljs-title">string</span></span><span> {
    </span><span><span class="hljs-keyword">if</span></span><span> (!</span><span><span class="hljs-title function_ invoke__">preg_match</span></span><span>(</span><span><span class="hljs-string">'/^[0-9]+$/'</span></span><span>, </span><span><span class="hljs-variable">$dec</span></span><span>)) {
        </span><span><span class="hljs-keyword">throw</span></span><span> </span><span><span class="hljs-keyword">new</span></span><span> </span><span><span class="hljs-built_in">InvalidArgumentException</span></span><span>(</span><span><span class="hljs-string">"非法十進制：<span class="hljs-subst">$dec</span></span></span><span>");
    }
    </span><span><span class="hljs-variable">$g</span></span><span> = </span><span><span class="hljs-title function_ invoke__">gmp_init</span></span><span>(</span><span><span class="hljs-variable">$dec</span></span><span>, </span><span><span class="hljs-number">10</span></span><span>);
    </span><span><span class="hljs-keyword">return</span></span><span> </span><span><span class="hljs-title function_ invoke__">gmp_strval</span></span><span>(</span><span><span class="hljs-variable">$g</span></span><span>, </span><span><span class="hljs-number">8</span></span><span>);
}
</span></span>

提示：GMP 擴展需在環境中啟用；如果不能使用GMP，可用bc或純PHP 實現逐位進制轉換。

4. 與base_convert()的對比

base_convert(string $num, int $from, int $to)也能實現進制轉換，例如：

 <span><span><span class="hljs-meta">&lt;?php</span></span><span>
</span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-title function_ invoke__">base_convert</span></span><span>(</span><span><span class="hljs-string">'17'</span></span><span>, </span><span><span class="hljs-number">8</span></span><span>, </span><span><span class="hljs-number">10</span></span><span>); </span><span><span class="hljs-comment">// "15"</span></span><span>
</span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-title function_ invoke__">base_convert</span></span><span>(</span><span><span class="hljs-string">'15'</span></span><span>, </span><span><span class="hljs-number">10</span></span><span>, </span><span><span class="hljs-number">8</span></span><span>); </span><span><span class="hljs-comment">// "17"</span></span><span>
</span></span>

區別與取捨：

base_convert()入參/出參都是字符串，更接近“文本進制轉換”的語義；
octdec()/decoct()更貼近“八進制?十進制”的專用API，語義明確，且octdec()直接返回int ，用起來順手；
面對超大整數時， base_convert()在部分場景會受實現與精度限制；要絕對穩妥，仍建議用gmp / bc 。

5. 小結

答案：可以。用octdec() （八進製字符串→十進制int ）與decoct() （十進制int →八進製字符串）即可實現雙向轉換。
生產實踐裡請注意：輸入校驗、負數處理、整數字面量前導0 的歧義、以及大整數的溢出問題。需要大數支持時，選擇gmp / bc等方案更穩健。