Large files merge sort

// Sort large files
	function countsLines($path){
		$fd = fopen($path,"r");
		$total=0;
		while(!feof($fd)){
			$total++;
			fgets($fd);
		}
		return $total;
	}
	$filePath = "./file.dat";
	function checkFiles($path,$rows=5000){
		$totalFiles = countsLines($path);
		$totalFiles=ceil($totalFiles/$rows);
		$files=[];
		$fd=fopen($path,"r");
		for($i=1;$i<=$totalFiles;$i++){
			$fileName=$i.".dat";
			$files[]=$fileName;
			$sd = fopen($fileName,"w+");
			$j=1;
			while(!feof($fd) && $j <= $rows){
				fputs($sd,fgets($fd));
				$j++;
			}
			fclose($sd);
		}
		fclose($fd);
		return $files;
	}
	function cSort($files){
		foreach ($files as $key => $file) {
			$content = file($file);
			$content = array_map(function($value){
				return trim($value);
			},$content);
			sort($content);
			$fd = fopen($file,"w+");
			foreach ($content as $key => $value) {
				$value=$value.PHP_EOL;
				fputs($fd,$value);
			}
		}
	}
	// merge array
	function mergeAllData($files){
		while(count($files) > 1){
			$fname1 = array_shift($files);
			$fname2 = array_shift($files);
			$newFileName = str_replace(".dat","", $fname1)."-".str_replace(".dat","",$fname2).".dat";
			$fd1=  fopen($fname1,"r");
			$fd2 = fopen($fname2,"r");
			$fd3 = fopen($newFileName,"w+");

			$line1 = fgets($fd1);
			$line2 = fgets($fd2);
			while(true){
				$line1 = intval($line1);
				$line2 = intval($line2);
				if($line1 < $line2){
					fputs($fd3,$line1.PHP_EOL);
					$line1 = fgets($fd1);
					if(feof($fd1)==true  || $line1 ===""){
						fputs($fd3,$line2.PHP_EOL);
						break;
					}
				}else{
					fputs($fd3,$line2.PHP_EOL);
					$line2=fgets($fd2);
					if(feof($fd2)==true || $line2 === ""){
						fputs($fd3,$line1.PHP_EOL);
						break;
					}

				}	
			}
			while(!feof($fd1)){
				$str =fgets($fd1);
				if($str != ""){
					fputs($fd3,$str);
				}
			}
			while(!feof($fd2)){
				$str =fgets($fd2);
				if($str != ""){
					fputs($fd3,$str);
				}
			}
			fclose($fd1);
			fclose($fd2);
			fclose($fd3);
			array_unshift($files, $newFileName);
		}
		
	}
	// Cutting
	$files = checkFiles($filePath);
	//merge
	cSort($files);
	mergeAllData ($ files);