Используем array_column и max
<?php
$raw = '{
"lastUpdateId":4409859389,
"bids":[
["2.13000000","6472.90000000"],
["2.12900000","50106.20000000"],
["2.12800000","63127.60000000"],
["2.12700000","31495.40000000"],
["2.12600000","41493.30000000"]
],
"asks":[
["2.13100000","24755.90000000"],
["2.13200000","86227.50000000"],
["2.13300000","58302.20000000"],
["2.13400000","61187.90000000"],
["2.13500000","39494.50000000"]
]
}';
$data = json_decode($raw, true);
$bids = array_column($data["bids"], 1);
$asks = array_column($data["asks"], 1);
echo "Max bid: " . max($bids) ,PHP_EOL;
echo "Max ask: " . max($asks) ,PHP_EOL;
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