Question

What is the fastest way to convert a simple array to an associative array in PHP so that values can be checked in the isset($array[$value])?

I.e. fastest way to do the following conversion:

$array = array(1, 2, 3, 4, 5);
$assoc = array();

foreach ($array as $i => $value) {
        $assoc[$value] = 1;
}

Answer 1

array_flip() is exactly doing that:

array_flip() returns an array in flip order, i.e. keys from trans become values and values from trans become keys.

Note that the values of trans need to be valid keys, i.e. they need to be either integer or string. A warning will be emitted if a value has the wrong type, and the key/value pair in question will not be flipped.

If a value has several occurrences, the latest key will be used as its values, and all others will be lost.

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But apart from that, there is only one type of array in PHP. Even numerical ("simple", as you call it) arrays are associative.