Yes, but not by magic. A standard black plastic box in full sun will dry out fast. The planters that survive are designed around four things: water storage, soil volume, heat control, and plant choice.
Choose a sub-irrigated or self-watering planter. These have a reservoir below the soil and a wick or fill tube. Roots draw moisture upward, so the surface stays damp longer. A 20–30 gallon box is better than a 5-gallon one because it holds more moisture and buffers heat. Light-colored, double-walled, or wood-lined planters stay cooler than thin dark plastic or metal.
Soil should be a coarse potting mix, not garden soil. Mix in compost and coconut coir for moisture retention, plus perlite or pumice for drainage. Top with 2–3 inches of mulch—bark, straw, or gravel—to slow evaporation. If you can, add a drip irrigation line on a timer or use clay ollas. A moisture sensor helps you avoid both drought and rot.
Plant drought-tolerant, sun-loving species: rosemary, lavender, sedum, portulaca, lantana, ornamental grasses, dwarf oleander, and bougainvillea in warm zones. In very hot cities, give roots afternoon shade by planting tall heat-lovers around the box or placing it where late sun is filtered.
No planter in full sun stays perfectly moist all day without some help. But a large sub-irrigated box with mulch, good soil, and timed drip irrigation can keep plants alive and healthy through a full-sun day—even in a heat island. If you want zero drying out, you need a reservoir plus automatic watering; if you can water daily, a large mulched planter with tough plants may be enough.