怎麼用function_exists函數區分內置函數和自定義函數？詳細方法解析

gitbox 2025-08-26

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</span><span><span class="hljs-comment">// Date: 2025-07-02</span></span><span>
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<h1>怎麼用function_exists函數區分內置函數和自定義函數？詳細方法解析</h1>

<p>在PHP開發過程中，我們經常會遇到需要判斷某個函數是否存在的情況。PHP內置了一個非常實用的函數 <code></span><span><span class="hljs-title function_ invoke__">function_exists</span></span><span>()

使用function_exists()判斷函數存在的基本示例

&lt;?php
</span><span><span class="hljs-keyword">if</span></span><span> (</span><span><span class="hljs-title function_ invoke__">function_exists</span></span><span>(</span><span><span class="hljs-string">'array_merge'</span></span><span>)) {
    </span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-string">'array_merge 函數存在'</span></span><span>;
} </span><span><span class="hljs-keyword">else</span></span><span> {
    </span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-string">'array_merge 函數不存在'</span></span><span>;
}
?&gt;

在這個例子中， array_merge是PHP的內置函數，通常都會返回存在。

為什麼不能直接用function_exists()區分內置函數和自定義函數？

雖然function_exists()可以檢測函數是否存在，但它本身無法告訴你這個函數是PHP內置的，還是用戶自定義的。它只關心函數名是否被註冊了。

換句話說， function_exists()對內置函數和自定義函數是“平等”的，只判斷是否存在，不區分來源。

那麼，怎麼區分內置函數和自定義函數？

要區分內置函數和自定義函數，可以結合get_defined_functions()函數使用。

該函數返回一個關聯數組，包含兩個鍵：

internal ：所有內置函數名數組
user ：所有用戶自定義函數名數組

示例代碼：

 &lt;?php
</span><span><span class="hljs-variable">$allFunctions</span></span><span> = </span><span><span class="hljs-title function_ invoke__">get_defined_functions</span></span><span>();

</span><span><span class="hljs-variable">$functionName</span></span><span> = </span><span><span class="hljs-string">'array_merge'</span></span><span>;

</span><span><span class="hljs-keyword">if</span></span><span> (</span><span><span class="hljs-title function_ invoke__">function_exists</span></span><span>(</span><span><span class="hljs-variable">$functionName</span></span><span>)) {
    </span><span><span class="hljs-keyword">if</span></span><span> (</span><span><span class="hljs-title function_ invoke__">in_array</span></span><span>(</span><span><span class="hljs-variable">$functionName</span></span><span>, </span><span><span class="hljs-variable">$allFunctions</span></span><span>[</span><span><span class="hljs-string">'internal'</span></span><span>])) {
        </span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-string">"<span class="hljs-subst">$functionName</span></span></span><span> 是內置函數";
    } </span><span><span class="hljs-keyword">elseif</span></span><span> (</span><span><span class="hljs-title function_ invoke__">in_array</span></span><span>(</span><span><span class="hljs-variable">$functionName</span></span><span>, </span><span><span class="hljs-variable">$allFunctions</span></span><span>[</span><span><span class="hljs-string">'user'</span></span><span>])) {
        </span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-string">"<span class="hljs-subst">$functionName</span></span></span><span> 是自定義函數";
    } </span><span><span class="hljs-keyword">else</span></span><span> {
        </span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-string">"<span class="hljs-subst">$functionName</span></span></span><span> 存在，但未分類";
    }
} </span><span><span class="hljs-keyword">else</span></span><span> {
    </span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-string">"<span class="hljs-subst">$functionName</span></span></span><span> 不存在";
}
?&gt;

完整示例：自定義函數與內置函數的區分

&lt;?php
</span><span><span class="hljs-comment">// 自定義函數示例</span></span><span>
</span><span><span class="hljs-function"><span class="hljs-keyword">function</span></span></span><span> </span><span><span class="hljs-title">my_custom_func</span></span><span>(</span><span><span class="hljs-params"></span></span><span>) {
    </span><span><span class="hljs-keyword">return</span></span><span> </span><span><span class="hljs-string">'这是自定義函數'</span></span><span>;
}

</span><span><span class="hljs-variable">$allFunctions</span></span><span> = </span><span><span class="hljs-title function_ invoke__">get_defined_functions</span></span><span>();

</span><span><span class="hljs-comment">// 檢查內置函數</span></span><span>
</span><span><span class="hljs-variable">$func1</span></span><span> = </span><span><span class="hljs-string">'strlen'</span></span><span>;
</span><span><span class="hljs-comment">// 檢查自定義函數</span></span><span>
</span><span><span class="hljs-variable">$func2</span></span><span> = </span><span><span class="hljs-string">'my_custom_func'</span></span><span>;
</span><span><span class="hljs-comment">// 檢查不存在函數</span></span><span>
</span><span><span class="hljs-variable">$func3</span></span><span> = </span><span><span class="hljs-string">'non_exist_func'</span></span><span>;

</span><span><span class="hljs-function"><span class="hljs-keyword">function</span></span></span><span> </span><span><span class="hljs-title">checkFunctionType</span></span><span>(</span><span><span class="hljs-params"><span class="hljs-variable">$funcName</span></span></span><span>, </span><span><span class="hljs-variable">$allFunctions</span></span><span>) {
    </span><span><span class="hljs-keyword">if</span></span><span> (</span><span><span class="hljs-title function_ invoke__">function_exists</span></span><span>(</span><span><span class="hljs-variable">$funcName</span></span><span>)) {
        </span><span><span class="hljs-keyword">if</span></span><span> (</span><span><span class="hljs-title function_ invoke__">in_array</span></span><span>(</span><span><span class="hljs-variable">$funcName</span></span><span>, </span><span><span class="hljs-variable">$allFunctions</span></span><span>[</span><span><span class="hljs-string">'internal'</span></span><span>])) {
            </span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-string">"<span class="hljs-subst">$funcName</span></span></span><span> 是內置函數\n";
        } </span><span><span class="hljs-keyword">elseif</span></span><span> (</span><span><span class="hljs-title function_ invoke__">in_array</span></span><span>(</span><span><span class="hljs-variable">$funcName</span></span><span>, </span><span><span class="hljs-variable">$allFunctions</span></span><span>[</span><span><span class="hljs-string">'user'</span></span><span>])) {
            </span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-string">"<span class="hljs-subst">$funcName</span></span></span><span> 是自定義函數\n";
        } </span><span><span class="hljs-keyword">else</span></span><span> {
            </span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-string">"<span class="hljs-subst">$funcName</span></span></span><span> 存在，但未分類\n";
        }
    } </span><span><span class="hljs-keyword">else</span></span><span> {
        </span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-string">"<span class="hljs-subst">$funcName</span></span></span><span> 不存在\n";
    }
}

</span><span><span class="hljs-title function_ invoke__">checkFunctionType</span></span><span>(</span><span><span class="hljs-variable">$func1</span></span><span>, </span><span><span class="hljs-variable">$allFunctions</span></span><span>);
</span><span><span class="hljs-title function_ invoke__">checkFunctionType</span></span><span>(</span><span><span class="hljs-variable">$func2</span></span><span>, </span><span><span class="hljs-variable">$allFunctions</span></span><span>);
</span><span><span class="hljs-title function_ invoke__">checkFunctionType</span></span><span>(</span><span><span class="hljs-variable">$func3</span></span><span>, </span><span><span class="hljs-variable">$allFunctions</span></span><span>);
?&gt;

運行以上代碼，你會得到：

 strlen 是內置函數
my_custom_func 是自定義函數
non_exist_func 不存在

總結

function_exists()能檢測函數是否已定義，但不能區分內置函數和自定義函數。
通過get_defined_functions()獲取系統所有函數，分別包含內置和用戶自定義函數。
結合這兩個函數，可以準確判斷一個函數是否存在，以及它是內置的還是自定義的。

掌握以上方法，能讓你在PHP開發中更加靈活地處理函數相關的邏輯，避免命名衝突或重複定義，提高代碼健壯性。