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

In programming, language constructs and built-in functions are often misinterpreted with each other due to the fact that both have more or less the same behavior. But they differ from each other in how the PHP interpreter interprets them. Every programming language is made up of tokens and structures that the corresponding parser of the language can recognize. This way, whenever a file is parsed, the parser understands their use and knows very well what to do with it, without having to study them further. These tokens and structures are known as language constructs . These are basically keywords that are part of the programming language. In other words, they form the syntax of the language. 
Here are some examples of language constructs:

echo() include() require() print() isset() die() 

Language constructs cannot be added to PHP environment through any plugins or libraries. They may or may not return any value, although most of them do not. Also, some of them do not need to use parentheses.Below are examples of using the language construct in PHP:
Example 1:

print ( ’ Monday ’ ); print ’Tuesday’ ; $str = ’Wednesday’ ; echo $str ;  
?> Exit:

Monday Tuesday Wednesday 

Example 2:
/ * PHP program to use unset
function * / $arr = array ( "1" = > "Amit" , "2" = > "Rajeev" , "3" = > "Mohit" , "4" = > "Manoj");  
// Use an undefined function for
// unset element unset ( $arr [ "2" ]);  
// Show array element print_r ( $arr );  
?> Exit:

Array ([ 1] = > Amit [3] = > Mohit [4] = > Manoj) 

On the other hand, built-in functions- they are blocks of code that are written in such a way that they can be used over and over again for a specific task. They are already included in the PHP installation package. It is these built-in features that make PHP an efficient scripting language. 
Some common built-in functions used in PHP:

json_encode() mail() explode() rand() curl_init() 

Built-in functions are comparatively slower than their language analogs. They have better code organization. They usually take input arguments and always return a value. Built-in functions usually consist of date, number, and string functions.Below are examples to illustrate the use of the built-in function in PHP:
Example 1:
/ * PHP date functions * / echo "Date and time is -" ; print date ( "j FY, gia" , time());  
?> Exit:

Date and time is - 26 February 2019, 12.22.pm 

Example 2:
/ * String functions in PHP * / $MyStr = "GeeksForGeeks" ; echo substr ( "GeeksForGeeks" , 5, 3). "" ; echo trim ( " GeeksForGeeks " ). "" ; echo str_replace ( "Geeks" , " Code " , " GeeksForGeeks " );  
?> Exit:

For GeeksForGeeks CodeForCode