A few utilities for functional programming in PHP.

functional.php

<?php

/**
 * Returns a callable which is the composition of all the arguments,
 * which should be callable themselves.
 */
function fn_compose( $fn ) {
	$funcs = func_get_args();

	$last = array_pop( $funcs );

	return function() use( $last, $funcs ) {
		$args = func_get_args();

		$r = call_user_func_array( $last, $args );

		foreach ( array_reverse( $funcs ) as $fn ) {
			$r = $fn( $r );
		}

		return $r;
	};
}

/**
 * Returns a callable with the arguments partially applied.
 */
function fn_partial( $fn ) {
	$args = array_slice( func_get_args(), 1 );

	return function() use ( $fn, $args ) {
		$final_args = array_merge( $args, func_get_args() );

		return call_user_func_array( $fn, $final_args );
	};
}

Example usage:

$d = [ [1, 2, 3], ['a', 'b', 'c', 'd'] ];

var_dump( array_map( fn_partial( 'implode', ' - ' ), $d ) );

var_dump( array_map( fn_compose( 'strlen', fn_partial( 'implode', '' ) ), $d ) );