将数组数组值存储在一个逗号分隔的字符串中

If you want a one-liner then this could work:

$csv = implode( ' , ', array_map( function( $a ){ return $a[ 'LeaseNumber' ]; }, $leaseArray ) );

expanded:

$csv = implode( ' , ', // step 2: implode on ' , ': space-comma-space
    array_map( // step 1: pass $lease array into array_map so that we can get a new array
        function( $a ){ return $a[ 'LeaseNumber' ]; }, // Accept each array in $a and only return the LeaseNumber. array_map will build a new array out of just these values
        $leaseArray // the array to be processed
    )
);

In essence, this is the same as:

$csv = implode( ' , ', array_column( $a, 'LeaseNumber' ) );

but array_map() allows you to transform the data before output/return if you need to.

If you want a one-liner then this is the way

$csv = implode(' , ', array_column($a, 'LeaseNumber'));

As I said in the comments, 1 line, 2 function calls.

One-liners are great but sometimes hard to read. If you want to stick with what you have here are some note

<?php 
$array = array(array("LeaseNumber" => "OL/2011/0343"), array("LeaseNumber"=> 184 ), array("LeaseNumber"=> "OL/2011/0118") , array("LeaseNumber"=> "OL/2016/1759"), array("LeaseNumber"=> "OL/2013/0858"), array("LeaseNumber"=> "OL/2012/0535"), array("LeaseNumber"=> "OL/2017/2208"), array("LeaseNumber"=> 2355));


$lease = array();

//Not equal to the count it starts at 1 not 0
for($i=0;$i<count($array); $i++){
    //lease is an array add to the index not overwrite
    $lease[] = $array[$i]['LeaseNumber'];
}

//You needed to finish with this
$csv = implode(", ",$lease);

echo $csv;

As an fyi switch to a foreach in this case makes life easier:

$lease = array();
foreach($array as $obj){
    $lease[] = $obj["LeaseNumber"];
}