Cloudflare API - Create DNS (NS) record using CURL PHP

I am working on adding records to my zone using curl (PHP); I want to add test.gen.ng as the DNS record with nameservers (ns1.test.com, ns2.test.com, ns3.test.com, ns4.test.com). I am able to add A records, CNAME records and others with the code at the bottom but not NS records.

{"success":false,"errors":[{"code":1004,"message":"DNS Validation Error","error_chain":[{"code":9102,"message":"content must be a string."}]}],"messages":[],"result":null}

I am having errors as shown above; sending the content below. Please, how do I send the content as required by cloudflare?

<?php
    /* Cloudflare.com | APİv4 | Api Ayarları */
   
   $apikey = $this->data["apikey"]; //'430255a2117147'; // Cloudflare Global API
    $email = $this->data["email"]; //'go@roswed.com'; // Cloudflare Email Adress
	
	
    $domain = $this->data["tld"];  //'gen.ng';  // zone_name // Cloudflare Domain Nam
    $zoneid = $this->data["zoneID"]; //'a203c506cbafc';
	
	
	
	$this->data["DomainName"]  = $this->get("loop_event10.row.Registry.sld");
	$this->data["NameServer1"] = $this->get("loop_event10.row.Registry.nameserver1");
	$this->data["NameServer2"] = $this->get("loop_event10.row.Registry.nameserver2");
	$this->data["NameServer3"] = $this->get("loop_event10.row.Registry.nameserver3");
	$this->data["NameServer4"] = $this->get("loop_event10.row.Registry.nameserver4");
	
	$this->data["TTL"] = 1;


    // A-record oluşturur DNS sistemi için.
    		$ch = curl_init("https://api.cloudflare.com/client/v4/zones/".$zoneid."/dns_records");
    		curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    		curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
    		curl_setopt($ch, CURLOPT_SSL_VERIFYPEER,false);
    		curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");                                                                     
    		curl_setopt($ch, CURLOPT_HTTPHEADER, array(
    		'X-Auth-Email: '.$email.'',
    		'X-Auth-Key: '.$apikey.'',
    		'Cache-Control: no-cache',
    	    // 'Content-Type: multipart/form-data; charset=utf-8',
    	    'Content-Type:application/json',
    		'purge_everything: true'
    		
    		));
    	
    		// -d curl parametresi.
    		$data = array(
    		
    			'type' => 'NS',
    			'name' => ''.$this->data["DomainName"].'',
				'content' => Array
				(
						'0' => $this->data["NameServer1"],
						'1' => $this->data["NameServer2"],
						'2' => $this->data["NameServer3"],
						'3' => $this->data["NameServer4"]
	
				),
				
    			'zone_name' => ''.$domain.'',
    			'zone_id' => ''.$zoneid.'',
    			'proxiable' => 'false',
    			'proxied' => false,
    			'ttl' => 1
    		);
    		
    		$data_string = json_encode($data);

    		curl_setopt($ch, CURLOPT_POST, true);
    		curl_setopt($ch, CURLOPT_POSTFIELDS, $data_string);	
    		//curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($data_string));

    		$sonuc = curl_exec($ch);

                 // If you want show output remove code slash.
		 // print_r($sonuc);

    		curl_close($ch);

			echo $sonuc;
?>

Says it right there doesn't it mate? "Content must be a string" and yet you're trying to send an array?