CreateTheInvoice.php

<?php
$invoice = new Invoice();

// ### Invoice Info
$invoice
	->setMerchantInfo( new MerchantInfo() )
	->setBillingInfo( array( new BillingInfo() ) )
	->setNote( "Thank you, customer!" )
	->setPaymentTerm( new PaymentTerm() )
	->setShippingInfo( new ShippingInfo() )
	->setTerms( "By paying your invoice you are agreeing ... set your terms here");

// ### Merchant Info
$invoice->getMerchantInfo()
	->setEmail( "your@email.com" )
	->setFirstName( "YourFirstName" )
	->setLastName( "YourLastName" )
	->setbusinessName( "Your Company LLC" )
	->setAddress( new Address() );

// The address used for creating the invoice
$invoice->getMerchantInfo()->getAddress()
	->setLine1( "1111 Somewhere Circle" )
	->setCity( "CityName" )
	->setState( "AK" )
	->setPostalCode( "12345" )
	->setCountryCode( "US" );

// ### Billing Information
// Set the email address for each billing
$billing = $invoice->getBillingInfo();
$billing[0]
	->setEmail( "person@beingbilled.com" );

// ### Items List

$items    = array();
$items[0] = new InvoiceItem();
$items[0]
	->setName( "Item Name" )
	->setQuantity( 1 )
	->setUnitPrice( new Currency() );

$items[0]->getUnitPrice()
	->setCurrency( "USD" )
	->setValue( $anitem['total'] );

if( $tax ) {
	$tax = new \PayPal\Api\Tax();
	$tax->setPercent( 4 )->setName( "Local Tax" );
	items[0]->setTax( $tax );
}
$invoice->getPaymentTerm()
	->setTermType( "DUE_ON_RECEIPT" );

$invoice->setItems( $items );

try {
	// ### Create Invoice
	// Create an invoice by calling the invoice->create() method
	// with a valid ApiContext (See bootstrap.php for more on `ApiContext`)
	$invoice->create( $apiContext );
} catch ( Exception $ex ) {
	// NOTE: PLEASE DO NOT USE RESULTPRINTER CLASS IN YOUR ORIGINAL CODE. FOR SAMPLE ONLY
	ResultPrinter::printError( "Create Invoice", "Invoice", null, $request, $ex );
	exit( 1 );
}