What is the difference between a language construct and a "built-in" function in PHP?

What is the difference between a language construct and a "built-in" function in PHP?

php function built-in language-construct

php

How to find a value in an array and remove it by using PHP array functions?

php arrays function built-in

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PHP array function that returns a subset for given keys

php arrays higher-order-functions built-in

php

How to override built-in PHP function(s)?

php built-in overriding

php

PHPUnit: Mocking get() results in "get() must take exactly 1 argument ..."

php phpunit overloading built-in

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