Because the properties for the object are predefined PHP no longer has to store the data in a hashtable, but instead can say that
$foois property 0,
$baris property 1,
$bazis property 2 and then just store the properties in a three-element C array. This means that PHP only needs one hashtable in the class that does the property-name to offset mapping and uses a memory-efficient C-array in the individual objects. Arrays on the other hand need the hashtable for every array.
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Why objects (usually) use less memory than arrays in PHP
Posted on | Freek Van der Herten