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    <p>When mapping an anonymous function over an anonymous array, there is no way to access the keys:</p> <pre><code>array_map( function($val) use ($foo) { /* ... */ }, array(key1 =&gt; val1, key2 =&gt; val2, /* ... */)); </code></pre> <p>array_reduce doesn't get access to the keys either. array_walk can access keys, but the array is passed by reference, which requires a layer of indirection.</p> <p>Some solutions are:</p> <h3>Array of pairs</h3> <p>This is bad, since we're changing the original array. Plus the boilerplate "array()" calls increase linearly with the length of the array:</p> <pre><code>array_map( function($pair) use ($foo) { list($key, $val) = $pair; /* ... */ }, array(array(key1, val1), array(key2, val2), /* ... */)); </code></pre> <h3>Temporary variable</h3> <p>We're acting on the original array, and the boilerplate is constant, but we can easily clobber an existing variable:</p> <pre><code>$i_hope_this_does_not_conflict = array(key1 =&gt; val1, key2 =&gt; val2, /* ... */); array_map( function($key, $val) use ($foo) { /* ... */ }, array_keys($i_hope_this_does_not_conflict), $i_hope_this_does_not_conflict); unset($i_hope_this_does_not_conflict); </code></pre> <h3>One-shot function</h3> <p>We can use function scope to prevent clobbering existing names, but have to add an extra layer of "use":</p> <pre><code>call_user_func( function($arr) use ($foo) { return array_map(function($key, $val) use ($foo) { /* ... */ }, array_keys($arr), $arr); }, array(key1 =&gt; val1, key2 =&gt; val2, /* ... */)); </code></pre> <h3>Multi-argument one-shot function</h3> <p>We define the function we're mapping in the original scope to prevent the "use" boilerplate):</p> <pre><code>call_user_func( function($f, $arr) { return array_map($f, array_keys($arr), $arr); }, function($key, $val) use ($foo) { /* ... */ }, array(key1 =&gt; val1, key2 =&gt; val2, /* ... */)); </code></pre> <h3>New function</h3> <p>The interesting thing to note is that our last one-shot function has a nice, generic signature and looks a lot like array_map. We might want to give this a name and re-use it:</p> <pre><code>function array_mapk($f, $arr) { return array_map($f, array_keys($arr), $arr); } </code></pre> <p>Our application code then becomes:</p> <pre><code>array_mapk( function($key, $val) use ($foo) { /* ... */ }, array(key1 =&gt; val1, key2 =&gt; val2, /* ... */)); </code></pre> <h3>Indirect Array Walk</h3> <p>When writing the above I'd ignored array_walk since it requires its argument to be passed by reference; however, I've since realised that it's easy to work around this using call_user_func. I think this is the best version so far:</p> <pre><code>call_user_func( 'array_walk', array(key1 =&gt; val1, key2 =&gt; val2, /* ... */), function($val, $key) use ($foo) { /* ... */ }); </code></pre>
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